Photoelectrocatalytic fluid analyte sensors including reference electrodes

ABSTRACT

Fluid analyte sensors include a photoelectrocatalytic element that is configured to be exposed to the fluid, if present, and to respond to photoelectrocatalysis of at least one analyte in the fluid that occurs in response to impingement of optical radiation upon the photoelectrocatalytic element. A semiconductor light emitting source is also provided that is configured to impinge the optical radiation upon the photoelectrocatalytic element. Related solid state devices and sensing methods are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/745,056,filed May 7, 2007, entitled Photoelectrocatalytic Fluid Analyte Sensorsand Methods of Fabricating and Using Same, which itself claims thebenefit of provisional Application Ser. No. 60/879,773, filed Jan. 11,2007, entitled Device and Method of Photoelectrocatalytic Sensing, andprovisional Application Ser. No. 60/905,489, filed Mar. 8, 2007,entitled Device Structures and Methods for PhotoelectrocatalyticSensing, the disclosures of all of which are hereby incorporated hereinby reference in their entirety as if set forth fully herein.

FIELD OF THE INVENTION

This invention relates to sensors and methods of fabricating and usingsensors, and more particularly to sensors that are configured to senseat least one analyte in a fluid, and methods of fabricating and usingsame.

BACKGROUND OF THE INVENTION

Sensors are increasingly being used in many consumer, commercial andother applications. For example, in the medical field, breath analysissensors may be used to assess overall metabolism, dieting efficiency,renal and hepatic health, ovulation, diabetes, the presence of a varietyof genetic disorders and/or many other effects. Moreover, environmentalexposure sensors may be used to detect, for example, volatile organiccompounds in a fluid.

Many different technologies have been used to provide sensors. Forexample, Non-Dispersive InfraRed absorption spectroscopy (NDIR) has beenused for carbon dioxide analysis, and various thermal catalysis gas andvapor sensing techniques also have been used. Unfortunately, thesetechnologies may have various shortcomings, especially in terms ofallowing portable/wearable, low power and/or low cost sensors.Environmental exposure monitors have employed PhotoIonization Detectors(PID), selective absorptive polymer capacitors and/or othertechnologies. Again, however, these technologies may present variousshortcomings, especially in terms of allowing portable/wearable, lowpower and/or low cost sensors.

SUMMARY OF THE INVENTION

Fluid analyte sensors, according to some embodiments of the presentinvention, comprise a photoelectrocatalytic element that is configuredto be exposed to the fluid, if present, and to respond tophotoelectrocatalysis of at least one analyte in the fluid that occursin response to impingement of optical radiation upon thephotoelectrocatalytic element. A semiconductor light emitting source isalso provided that is configured to impinge the optical radiation uponthe photoelectrocatalytic element.

In some embodiments, the photoelectrocatalytic element comprises aphotoelectrocatalytic layer and at least one conductive contactelectrically connected thereto. In other embodiments, thephotoelectrocatalytic element comprises a plurality of spaced apartphotoelectrocatalytic layers and a plurality of spaced apart conductivecontacts electrically connected thereto. In some embodiments, areference electrode and/or a charge balancing electrode also may beprovided. In still other embodiments, a substrate is provided, and thephotoelectrocatalytic element and the semiconductor light emittingsource are at least partially monolithically integrated with thesubstrate.

In some embodiments, the photoelectrocatalytic layer comprises aplurality of layers of a given photoelectrocatalytic material having atleast two different impurities therein. In other embodiments, aplurality of layers of different photoelectrocatalytic materials areprovided. In some embodiments, the photoelectrocatalytic layer comprisesoxide, carbide, nitride, arsenide, phosphide, sulfide and/or antimonidephotoelectrocatalytic compounds and/or metal oxide(s), metal nitride(s),metallic compounds and/or semimetallic compounds thereof. Moreover, inyet other embodiments, the photoelectrocatalytic element is doped withdeep level impurities, and the electro-optical source is a visibleand/or InfraRed (IR) Light Emitting Diode (LED). In still otherembodiments an UltraViolet (UV) LED is used. In still other embodiments,the photoelectrocatalytic film comprises a metal oxide, and thesemiconductor light emitting source comprises an UltraViolet (UV) LED.In yet other embodiments, laser diode(s) may be used.

In some embodiments, the photoelectrocatalytic element and thesemiconductor light emitting source include at least one commonelectrical contact. Moreover, in other embodiments, the semiconductorlight emitting source includes a passivation layer, and thephotoelectrocatalytic element is at least partially on the passivationlayer. In still other embodiments, the at least one conductive contactcomprises at least two interdigitated conductive contacts.

In some embodiments, the fluid comprises a liquid and/or a gas, and theanalyte can comprise a pollutant, contaminant and/or a component of thefluid. In still other embodiments, the fluid comprises respired gas, andthe analyte comprises a component of the respired gas. In still otherembodiments, the fluid comprises flowing air such as air in an HVACsystem.

In some embodiments, the photoelectrocatalytic element is configured torespond to photoelectrocatalysis of at least one analyte in the fluid bychanging a conductivity, capacitance, inductance, impedance, net charge,optical property and/or mechanical property thereof, in response toimpingement of optical radiation upon the photoelectrocatalytic element.Moreover, in other embodiments of the present invention, thephotoelectrocatalytic element may include a transistor, a capacitor, amicroelectromechanical structure, a diode, a resistor, a semiconductorswitch, an amorphous structure, a nanostructure, a piezoelectricstructure, a surface acoustic wave structure and/or a heating element.

Other embodiments of the present invention may add additional elementsto the fluid analyte sensors to accomplish various results. For example,a photodetector may be added to detect the optical radiation that isemitted by the semiconductor light emitting source and/or opticalradiation emitted by the photoelectrocatalysis. A controller may also beprovided that is configured to selectively energize the semiconductorlight emitting source and to detect a response to thephotoelectrocatalysis of at least one analyte in the fluid that occursin response to impingement of optical radiation upon thephotoelectrocatalytic element. In yet other embodiments, the controllermay be configured to repeatedly modulate the semiconductor lightemitting source and to detect an electrical response of the at least onephotoelectrocatalytic element in response thereto. In still otherembodiments, a wireless transmitter may be added that is responsive tothe controller, to wirelessly transmit results of fluid analyte sensing.

In some embodiments, a monitor also may be added that is configured tomonitor an electrical, electromagnetic, mechanical, acoustic and/orthermal response to the at least one analyte in the fluid, if present,resulting from the photoelectrocatalysis of the at least one analyte inresponse to impingement of optical radiation upon thephotoelectrocatalytic element. In other embodiments, a monitor may beconfigured to monitor energy of the at least one analyte in the fluid,if present, resulting from photoelectrocatalysis of the at least oneanalyte in response to impingement of optical radiation upon thephotoelectrocatalytic element. In still other embodiments, the monitoris configured to monitor energy as one or more forms of optical energyof the at least one analyte in the fluid.

Solid state devices according to other embodiments of the presentinvention can comprise a photoelectrocatalytic element, a semiconductorlight emitting source and a housing that is configured to position thephotoelectrocatalytic element and the semiconductor light emittingsource relative to one another, such that the semiconductor lightemitting source impinges optical radiation on the photoelectrocatalyticelement upon electrical energization of the semiconductor light emittingsource. These solid state devices may be used for sensing and/or otherapplications.

In some embodiments, the photoelectrocatalytic element comprises aphotoelectrocatalytic layer and an electrical contact layer thereon, andthe semiconductor light emitting source comprises a light emittingdiode. In yet other embodiments, the photoelectrocatalytic layerincludes first and second opposing faces. The electrical contact layeris on the first face. The light emitting diode impinges opticalradiation on the first face and/or on the second face. Any of the otherembodiments of the invention described herein may also be included inthese solid state devices.

Sensing methods according to other embodiments of the present inventioncomprise energizing a semiconductor light emitting source to impingeoptical radiation upon a photoelectrocatalytic element that isconfigured to be exposed to a fluid, if present, and detecting aresponse of the photoelectrocatalytic element in response to theenergizing of the semiconductor light emitting source. Again, any of theother embodiments described herein may be included in these sensingmethods. Moreover, these methods can also include repelling chargedanalyte from the photoelectrocatalytic element and/or biasing thephotoelectrocatalytic element to reduce carrier recombination.

Finally, it will be understood by those having skill in the art that anyof the embodiments described herein may be combined in variouscombinations and subcombinations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 summarizes a list of health factors, with associated analytes,that can be monitored through the breath.

FIGS. 2 and 3 are block diagrams of fluid analyte sensors according tovarious embodiments of the present invention.

FIG. 4 is a block diagram of a solid state device according to variousembodiments of the present invention.

FIG. 5 is a flowchart of operations that may be performed to providesensing according to various embodiments of the present invention.

FIG. 6 conceptually illustrates photoelectrocatalytic sensing devicesand methods according to various embodiments of the present invention.

FIG. 7 is a block diagram of a photoelectrocatalytic sensor according tovarious embodiments of the present invention.

FIG. 8 is a cross-sectional view and FIG. 9 is a top view of aphotoelectrocatalytic sensor according to other embodiments of thepresent invention.

FIG. 10 is a cross-sectional view of a photoelectrocatalytic sensoraccording to still other embodiments of the present invention.

FIG. 11 conceptually illustrates photoelectrocatalytic sensing devicesand methods according to other embodiments of the present embodiment.

FIG. 12 illustrates photoelectrocatalytic sensors according to stillother embodiments of the present invention, as packaged in a wirelesscommunications module for portable analyte monitoring according tovarious embodiments of the present invention.

FIG. 13A is a photograph of an experimental apparatus that may be usedto demonstrate photoelectrocatalytic sensing using a UV LED according tovarious embodiments of the present invention.

FIG. 13B is a circuit diagram of the experimental apparatus of FIG. 13A,and graphically illustrates differential photoelectrocatalytic responsesas a function of ethanol concentration that was observed.

FIG. 13C schematically illustrates another experimental apparatus thatmay be used to demonstrate photoelectrocatalytic sensing of variousvolatile organic compounds using an InGaZnO MSM photoelectrocatalyticelement according to various embodiments of the present invention.

FIG. 13D graphically illustrates response of the photoelectrocatalyticelement as a function of concentration of ethanol, methanol and acetonethat was observed during the experiment of FIG. 13C.

FIGS. 14A-14C graphically illustrate fundamental physics of aphotoelectrocatalytic effect, where an analyte is oxidized according tovarious embodiments of the present invention.

FIGS. 15A-15C graphically illustrate fundamental physics of aphotoelectrocatalytic effect, where an analyte is reduced according tovarious embodiments of the present invention.

FIGS. 16A-16C graphically illustrate fundamental physics of aphotoelectrocatalytic effect, where sub-band gap impurities reduce ananalyte at the catalyst surface, such that the analyte can be sensedelectrically via sub-band gap photoexcitation according to variousembodiments of the present invention.

FIG. 17 is a cross-sectional view and FIG. 18 is a top view of aphotoelectrocatalytic sensor according to still other embodiments of thepresent invention.

FIG. 19 is a top view of a photoelectrocatalytic sensor according toother embodiments of the present invention, illustrating interdigitatedstructures according to various embodiments of the present invention.

FIG. 20 is a cross-sectional view of a photoelectrocatalytic sensoraccording to still other embodiments of the present invention, where thephotoelectrocatalytic electrodes are arrayed directly on a backside of atransparent substrate of a flip-chip LED.

FIG. 21 is a cross-sectional view of a photoelectrocatalytic sensoraccording to various embodiments of the present invention, where thephotoelectrocatalytic material is integrated into a passivation orp-contact of an epi-up LED.

FIG. 22 is a cross-sectional view of a photoelectrocatalytic sensoraccording to other embodiments of the present invention, where thephotoelectrocatalytic electrodes are arranged as a capacitor.

FIG. 23 is a cross-sectional view of a photoelectrocatalytic sensoraccording to other embodiments of the present invention, where thephotoelectrocatalytic electrodes are arranged as a structure that canmeasure charge, photocurrent, capacitance and/or impedance in the samedevice.

FIGS. 24A and 24B are cross-sectional views of photoelectrocatalyticsensor configurations according to various embodiments of the presentinvention, where the photoelectrocatalytic electrodes are arranged on apiezoelectric material.

FIGS. 25A and 25B are top views of photoelectrocatalytic sensorsaccording to still other embodiments of the present invention, where thephotoelectrocatalytic electrode is integrated in the active region of aSurface Acoustic Wave (SAW) device and the photoelectrocatalyticelectrode is integrated into the driving electrodes of a SAW device,respectively.

FIGS. 26A and 26B are cross-sectional views of photoelectrocatalyticsensors according to various embodiments of the present invention thatare combined with thermocatalysis according to various embodiments ofthe present invention.

FIG. 27 is a side view of a photoelectrocatalytic sensor according toother embodiments of the present invention that integratesphotoelectrocatalytic sensing and optical sensing.

FIGS. 28A and 28B illustrate band diagrams for photoelectrocatalyticsensing of a reducible analyte.

FIG. 28C is a circuit diagram for embodiments of FIGS. 28A and 28B.

FIG. 29 is a cross-sectional view of a photoelectrocatalytic sensoraccording to various embodiments of the present invention, where aphotoelectrocatalytic material is integrated into one or more layers ofa gate of a field effect transistor (FET).

FIG. 30 is a cross-sectional view of a photoelectrocatalytic sensoraccording to various embodiments of the present invention, that includesa charge balancing electrode.

FIG. 31 is a cross-sectional view of a photoelectrocatalytic sensoraccording to various embodiments of the present invention, that includesa plurality of charge balancing electrodes.

FIGS. 32A and 32B are cross-sectional views of a photoelectrocatalyticsensor according to various embodiments of the present invention,wherein the photoelectrocatalytic material is integrated into one ormore layers of a diode.

DETAILED DESCRIPTION

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” (and variants thereof) whenused herein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. In contrast,the term “consisting of” (and variants thereof) when used in thisspecification, specifies the stated features, integers, steps,operations, elements, and/or components, and precludes additionalfeatures, integers, steps, operations, elements and/or components.Moreover, the term “consisting essentially of” when used in thespecification, specifies the stated number of features, integers, steps,operations, elements and/or components, and precludes additionalfeatures, integers, steps, operations, elements and/or components,except for insubstantial amounts of impurities or other materials thatdo not materially affect the basic and novel characteristics of thestated features, integers, steps, operations, elements and/orcomponents.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement (and variants thereof), it can be directly on or extend directlyonto the other element or intervening elements may also be present. Incontrast, when an element is referred to as being “directly on” orextending “directly onto” another element (and variants thereof), thereare no intervening elements present. It will also be understood thatwhen an element is referred to as being “connected” or “coupled” toanother element, it can be directly connected or coupled to the otherelement or intervening elements may be present. In contrast, when anelement is referred to as being “directly connected” or “directlycoupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “lateral” or “vertical” may be used herein to describe arelationship of one element, layer or region to another element, layeror region as illustrated in the figures. It will be understood thatthese terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the figures.

Embodiments of the invention are described herein with reference tocross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be exaggeratedfor clarity. Additionally, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, embodiments of theinvention should not be construed as limited to the particular shapes ofregions illustrated herein but are to include deviations in shapes thatresult, for example, from manufacturing. For example, variations insurface or interface roughness, porosity, morphology and/orstoichiometry differences due to materials growth/deposition techniquesare included. In another example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a discretechange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place. Thus, the regions illustrated in the figures are schematicin nature and their shapes are not intended to illustrate the actualshape of a region of a device and are not intended to limit the scope ofthe invention.

The present invention is described below with reference to blockdiagrams and/or flowchart illustrations of methods and/or apparatus(systems) according to embodiments of the invention. It is understoodthat a block of the block diagrams and/or flowchart illustrations, andcombinations of blocks in the block diagrams and/or flowchartillustrations, can embody apparatus/systems (structure), means(function) and/or steps (methods) for implementing the functions/actsspecified in the block diagrams and/or flowchart block or blocks.

It should also be noted that in some alternate implementations, thefunctions/acts noted in the blocks may occur out of the order noted inthe flowcharts. For example, two blocks shown in succession may in factbe executed substantially concurrently or the blocks may sometimes beexecuted in the reverse order, depending upon the functionality/actsinvolved. Moreover, the functionality of a given block of the flowchartsand/or block diagrams may be separated into multiple blocks and/or thefunctionality of two or more blocks of the flowcharts and/or blockdiagrams may be at least partially integrated.

Some embodiments of the invention can provide devices and/or methods forsensing various analytes within a fluid via the photoelectrocatalytic(PEC) effect. Some embodiments can integrate a solid state opticalemitter with one or more photoelectrocatalytic films for sensing variousanalytes within a fluid and/or other applications. In some embodiments,the solid state optical emitter is an UltraViolet (UV) Light EmittingDiode (LED), and the photoelectrocatalytic films include metal oxidefilms in an array. These embodiments can allow multiple species in fluidmixtures to be qualified and quantified independently in real time bythe photoelectrocatalytic response at each electrode. In otherembodiments, the photoelectrocatalytic array fluid sensor may be appliedtowards respiration monitoring, for monitoring overall metabolism and/ordiagnosing various health conditions through the breath.

In 1983, Manolis reported on the broad diagnostic potential of breathanalysis in medical applications, citing roughly 200 compounds, mostlyVolatile Organic Compounds (VOCs), present in the breath of normal humanpatients. In this report, Manolis highlighted that breath monitoring canbe used to assess overall metabolism, dieting efficacy, renal andhepatic health, ovulation, diabetes and/or the presence of a variety ofgenetic disorders as illustrated in FIG. 1. Breath monitoring may haveone or more of the following advantages over competing approaches:breath samples can be easy, fast, and cost effective to collect;respired gas monitoring can often provide the same information asarterial blood gas testing, but with a noninvasive, real-time monitoringapproach; breath samples generally are chemically less complex thanblood or urine, which can reduce or preclude the need for costly samplepreparation prior to chemical analysis; and chemical breath analysis canprovide a direct method of monitoring respiratory function that may notbe easily attainable by other means.

At the time of Manolis' report, the instrumentation for breath analysisgenerally was bulky and expensive in comparison with blood and urineanalysis. Today, a variety of potential solutions exist for measuringCO₂ levels and low-density VOCs, but these technologies may not providea holistic solution for monitoring many or all relevant exhaled gasesand vapors in a single, portable, compact, battery-powered device. Sucha device may be used not only for hospital patient monitoring, but alsofor paramedics, physical therapy clinics, and home health monitoring.

For example, currently two technologies may aim to provide low-cost,portable respiration monitoring: Non-Dispersive InfraRed absorptionspectroscopy (NDIR) for carbon dioxide analysis, and thermal catalysisgas and vapor sensing. An NDIR-based CO₂ monitor can be used to measurecarbon dioxide via infrared absorption. Similarly, thermal catalysis canbe applied towards measuring carbon dioxide and other gases via hightemperature catalytic reactions. However, infrared absorption may onlybe effective for carbon dioxide sensing, and thermal catalysis sensorsmay use excessively high input power and may be too hot for wearablesensor applications. One current portable gas monitor solution combinesthe NDIR approach with an electrochemical approach to cover a widerrange of gaseous species, but these incompatible technologies maydramatically increase system size, boost power demands and/or complicatemaintenance. Thus, these technologies may be characterized bypotentially serious shortcomings in the context of portable/wearablehealth monitors.

Of the presently available technologies, thermal catalysis may be themost flexible, as a variety of gases can be sensed, in some casesselectively from other gases in a mixture, by monitoring the electroderesponse at high temperatures. In these sensors, patterned metal-oxideelectrodes serve as thermal catalytic films, and at elevatedtemperatures catalyzed gas reactions are measured as changes in the bulkor surface conductivity within the catalytic films. However, the hightemperatures that are used may make these sensors unsuitable forwearable use. Furthermore, the high temperatures can negatively affectsensor sensitivity, selectivity, lifetime, initial turn-on time,response time, power consumption and/or reliability.

In contrast, some embodiments of the invention can catalyze gasreactions in a catalytic film by employing photocatalysis as opposed tothermal catalysis. For example, through the known photocatalytic effectof titanium dioxide, organic vapors are adsorbed onto the titania(titanium dioxide) surface and dissociated via photoelectric radicalformation. Similarly, other gaseous species, such as carbon monoxide andcarbon dioxide, can be oxidized or reduced on the titania surface. Byintegrating electrodes with the titania film, the electrical propertiesof the film in response to photoelectrocatalyzed gaseous species can bemonitored and related to the concentration of the species, in someembodiments of the invention.

In other embodiments, a photoelectrocatalytic array fluid sensor may beapplied to analyte monitoring, for monitoring volatile organicchemicals, airborne pollutants, liquid pollutants, and the like. Inparticular, despite the growing commercial market for affordable,low-profile environmental exposure monitors, commercially availablesolutions do not appear to satisfy the desires of end users. Incontrast, some embodiments of the invention can provide compact,wearable, low-power photoelectrocatalytic sensors and can combinemetal-oxide photoelectrocatalytic sensors with commercially available UVLEDs, which can potentially satisfy consumer needs for broad sensorfunctionality, virtually unnoticeable size and weight and/or fewerrecharges per week.

To potentially satisfy commercial markets for personal environmentalexposure monitors, it may be desirable for an environmental sensormodule to satisfy one or more of the following criteria. In particular,the sensor module should be: compact (e.g., less than about 10 cm³); lowpower (e.g., less than about 10 mW average power, suitable for miniaturerechargeable batteries); low-profile (e.g., virtually unnoticed by theuser); multifunctional (able to monitor a variety of airborne species inthe same module); affordable; sufficiently responsive in turn-on andturn-off time; robust (indoors and outdoors); reusable; and/orlong-lasting (e.g., up to 2 years or more). In today's marketplace, avariety of commercially available products may provide affordablesolutions for monitoring VOCs. However, although many of these solutionsmay be well developed for industrial use, these technologies may not bereadily usable for personal, wearable monitoring of VOC exposure.

For example, commercial Photo-Ionization Detectors (PIDs) candemonstrate very low detection limits of volatile organic vapors (partsper billion), such as acetone, toluene, benzene, octane, etc. However,even the most compact of these sensors generally is much too large andcostly, may use high operating power (e.g. more than about 100 mW), andmay operate over a narrow operating temperature range. Conventional tinoxide VOC sensors, a potentially affordable alternative to PIDs, mayindeed be compact, robust, low-profile and/or sensitive to various gases(VOCs, ozone, CO, etc.), but they also may operate at high operatingtemperatures (for thermocatalysis), may lack vapor selectivity, and/ormay use operating powers that are too high for multi-day operation usinga small rechargeable battery.

Another sensing technology is the selective absorptive polymercapacitor. Researchers are currently testing compact chemical sensingmodules via polymer-filled capacitor technology. These sensors may becompact, may use low (sub-milliwatt) operating power, and may be easilyintegrated into compact integrated circuits. However, concerns mayremain with long-term reliability and/or sensor selectivity duringwearable use. In particular, polymer materials can be especiallysensitive to humidity, temperature and/or aerosolized interferents,especially over long-term operation. Further, it may be difficult tointegrate absorptive polymer VOC sensors with sensors for ozone, carbonmonoxide, and other gases on the same sensor module. Additionally,absorptive polymers may pose concerns of response time, saturationand/or desorption time, since adsorption of analyte into the filmgenerally is part of the sensing process.

In contrast, some embodiments of the invention can integrate UV LEDswith metal oxide catalysts, for a compact photoelectrocatalytic device.For monitoring VOCs, one or more of the following potential advantagesmay be provided by a photoelectrocatalytic device according to someembodiments of the invention, over standard thermal catalysis approach:a wider variety of vapors can be sensed using the same catalyst; hightemperatures (and thus high operating powers) may not be needed forcatalysis; because surface adsorption may be dominated by the uniquephotoelectrochemical dynamics of each gaseous species, as opposed to themore ambiguous thermodynamics of each species, greater selectivity maybe afforded using the same photocatalytic film; by modulating the UV LEDphoto-excitation source, much higher sensitivity (e.g., about100-10,000×) may be achievable through a lock-in detection approach;and/or higher detection speeds can be realized.

Moreover, some embodiments of the invention may provide for: measuringmultiple gases and vapors simultaneously using multiple electrodes;implementing multiple catalysts to allow improved selectivity amongvarious species; integrating commercially available UV LEDs within a PECarray; and/or using PEC sensors in a portable respirometer formonitoring health conditions, diseases and/or health factors in thebreath. Yet other embodiments can provide portable PEC sensing devicesand methods for personal environmental exposure monitoring of volatileorganic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), ozoneand/or other airborne pollutants and toxins.

FIG. 2 is a block diagram of a fluid analyte sensor according to variousembodiments of the present invention. Referring to FIG. 2, a fluidanalyte sensor 200 includes a photoelectrocatalytic (PEC) element 210that is configured to be exposed to a fluid 220, if present, and torespond to photoelectrocatalysis of at least one analyte in the fluidthat occurs in response to impingement of optical radiation 230 upon thephotoelectrocatalytic element 210. A semiconductor light emitting source240 is configured to impinge the optical radiation 230 upon thephotoelectrocatalytic element 210. It will be understood by those havingskill in the art that the arrangement of the elements in FIG. 2 ismerely illustrative, so that the photoelectrocatalytic element 210 maybe exposed to the fluid on one or both faces thereof, and thesemiconductor light emitting source 240 may impinge the opticalradiation 230 on one or both faces of the photoelectrocatalytic element,which may be the same face and/or a different face as that which isexposed to the fluid 220. Various arrangements of photoelectrocatalyticelements 210 and semiconductor light emitting sources 240 in fluidanalyte sensors 200 will be described below. Moreover, many differentembodiments of the photoelectrocatalytic elements 210 and semiconductorlight emitting sources 240 themselves will also be described.

FIG. 3 is a block diagram of fluid analyte sensors 300 according tovarious other embodiments of the present invention. As shown in FIG. 3,a photoelectrocatalytic element 210 may include a photoelectrocatalyticlayer 310 and one or more conductive contacts 320 electrically connectedthereto. Various configurations of photoelectrocatalytic layers 310 andcontacts 320 may be provided, as will be described below. For example, aplurality of spaced apart photoelectrocatalytic layers 310 and aplurality of spaced apart conductive contacts 320 electrically connectedthereto may be provided as shown in FIG. 3. In other embodiments,contactless sensing may be employed so that conductive contacts 320 neednot be used. In still other embodiments, the photoelectrocatalyticelement 210 can include other regions or structures.

A substrate, described in detail below, may be used to maintain thephotoelectrocatalytic layers 310 in spaced apart relationship. Thephotoelectrocatalytic layers 310 may comprise a plurality of layers of agiven photoelectrocatalytic material having at least two differentimpurities therein and/or a plurality of different photoelectrocatalyticmaterials. In still other embodiments, a single photoelectrocatalyticlayer 310 may include various regions therein that provide at least twodifferent impurities therein and/or at least two differentphotoelectrocatalytic materials therein. Many specific examples will beprovided below. Moreover, the physical arrangement of thephotoelectrocatalytic elements 210 and the semiconductor light emittingsource 240 may vary considerably, as described in greater detail below.

FIG. 3 also illustrates a controller 330, also referred to herein as“electronics,” that may be used to receive the signals from theconductive contacts 320, and may also be used to energize thesemiconductor light emitting source 240. Various embodiments ofcontrollers will also be described in detail below. Moreover, many otherelements may be added to the basic configurations of FIGS. 2 and 3, aswill be described in greater detail below.

FIG. 4 illustrates a mechanical construction of a solid state device 400according to various embodiments of the invention. These embodiments ofsolid state devices 400 include a photoelectrocatalytic element 210 anda semiconductor light emitting source 240. A housing 410 is configuredto position the photoelectrocatalytic element 210 and the semiconductorlight emitting source 240 relative to one another, such that thesemiconductor light emitting source impinges optical radiation on thephotoelectrocatalytic element 210 upon electrical energization of thesemiconductor light emitting source 240. It will be understood by thosehaving skill in the art that the housing block 410 is used toconceptually illustrate any housing that is configured to position thephotoelectrocatalytic element 210 and the semiconductor light emittingsource 240 relative to one another, and includes single-piece ormulti-piece housings that are configured to position thephotoelectrocatalytic element 210 and the semiconductor light emittingsource 240 relative to one another in any orientation, includingadjacent or touching one another, as long as the semiconductor lightemitting source impinges optical radiation on the photoelectrocatalyticelement 210 (directly and/or via one or more optical elements) uponelectrical energization of the semiconductor light emitting source 240.The photoelectrocatalytic element 210 may be exposed to a fluid 220 toprovide a fluid analyte sensor. However, devices 400 may also be used inapplications other than fluid analyte sensing.

FIG. 5 is a flowchart of operations that may be performed to providesensing according to various embodiments of the present invention. Inparticular, as illustrated in FIG. 5, at Block 510, a semiconductorlight emitting source is energized to impinge optical radiation upon aphotoelectrocatalytic element. At Block 520, a response of thephotoelectrocatalytic element is detected, in response to the energizingof the semiconductor light emitting source. Various other operations maybe performed to process the detected response, to transmit the detectedresponse and/or the processed results thereof, and/or to provide otherfunctionality, as will be described in detail below.

Some embodiments of the invention may provide fluid analyte sensorscapable of identifying and quantifying multiple species within a fluidin real time. The fluid analyte sensors can combine semiconductoroptical emitters with photoelectrocatalytic elements, enabling alow-cost, low-power, low-profile, portable (in some embodimentsbattery-powered) and/or real-time gas monitor. In some embodiments, afluid analyte sensor operates as a respiration monitor capable ofidentifying and/or quantifying multiple respired gases and vaporssimultaneously and noninvasively in real time. In contrast with existinggas and vapor sensing products, some embodiments of the invention canselectively sense, quantify, and/or qualify O₂, CO₂, N₂ and/or variousvolatile organic chemicals simultaneously using the same monolithicdevice. This can enable the rapid assessment of various health factorswith a simple compact device, including cardiopulmonary status, renalstatus, hepatic health, etc.

Some embodiments of the invention can operate by monitoring theelectrochemical response to the photocatalytic effect of titania,zirconia and/or various other metal oxides, in the presence of respiredgases and vapors. For example, as shown in FIG. 6, with the adsorptionof water vapor at the titania surface, ultraviolet light will generatehydroxide radicals capable of oxidizing volatile organic compounds(VOCs) at the surface. Similarly, oxygen adsorbed under UV excitationgenerates superoxide anions which also can oxidize VOCs. This results inchanges in the conductivity of the titania film in response to theanalyte absorbed and ionized at the surface. According in someembodiments, the unique photoelectrocatalytic signatures for variousrespired gases (oxygen, carbon dioxide, nitrogen and/or the like) andVOCs (ethanol, methylene chloride, benzene, acetone, xylene, isopropanoland/or the like) can be monitored in real time, without needing hightemperatures. In some embodiments of the invention, analyte selectivitymay be achieved by monitoring the transient electrical response that canbe unique to each ionized species. In other embodiments, analyteselectivity may be achieved by monitoring and comparing the electricalresponse from multiple electrodes having preferentially selectivecatalysts, as illustrated in FIG. 6. Other techniques for achievinganalyte selectivity will be described in detail below. Monitoring VOCsin exhaled breath can be used to detect and diagnose various healthfactors and diseases, as was illustrated in FIG. 1.

Conventionally, the photocatalytic effect of titania has been appliedtowards the biological sterilization of air and water. Inphotoelectrocatalytic sensing according to some embodiments of theinvention, the photocatalyzed byproducts of gas and vapor oxidation arenot simply discarded but rather are monitored electrically asillustrated by the schematic voltmeter symbols (V) in FIG. 6. Theoxidized byproducts may be adsorbed onto (and/or partially absorbedinto) the titania film, proportionately changing its conductivity,capacitance and/or other property, and the resultingphotoelectrocatalytic response is measured electrically between two ormore conductive (e.g., metallic) contacts. More specifically,photoionized electron-hole pairs (E− and H+) selectively ionize exhaledgases (O₂, H₂O₂, CO₂, organic vapors, etc.) adsorbed near the surfacesof the titania photocatalysts. The adsorbed radicals temporarily alterthe titania conductivity, measured electrically before desorption. Thiscan provide a convenient tool for monitoring the presence and/orrelative intensity of adsorbed gases and vapors. Depending on thestoichiometry and morphology of the metal oxide film, different gasescan be selectively adsorbed and detected.

The species of the photocatalyzed vapor can be qualified by monitoringthe time-dependent electrical response, measuring the current-voltageprofile and/or by a selective (or quasi-selective) photocatalyticreaction. Similarly, the volumetric concentration of the analyte can bemonitored by the concentration-dependent intensity of the electricalresponse at the photocatalytic electrodes (i.e., thephotoelectrocatalytic response to the analyte). The temporal responsemay be governed primarily by the adsorption/desorption rate of thegaseous species within the metal oxide film, and this in turn can becontrolled to some degree by the thickness and/or morphology of thefilm. For example, thicker films and rougher surfaces may typicallyresult in a longer absorption/desorption rate.

For monitoring respired gases and vapors, one or more of the followingpotential advantages may be provided by a photoelectrocatalytic approachaccording to some embodiments of the invention, as opposed to thestandard thermal catalysis approach:

-   -   A wider variety of vapors can be sensed using the same catalyst;    -   High temperatures (and thus high input powers) are not needed to        catalyze a reaction;    -   Because surface adsorption may be dominated by the potentially        unique photoelectrochemical dynamics of each gaseous species, as        opposed to the more ambiguous thermodynamics of each species,        greater selectivity may be afforded using the same        photocatalytic film;    -   By modulating the photo-excitation source, much higher        sensitivity (e.g., 100-10,000 times) may be achievable through a        lock-in detection approach;    -   Millisecond-level detection speeds can be realized, as opposed        to timescales of seconds and minutes. This speed may be        especially problematic for traditional absorptive dielectric        sensors and high-temperature catalytic sensors;    -   Initial startup does not require a heating up time duration, so        that sensing can begin quickly, e.g., within milliseconds. This        may be especially useful in intermittent polling applications,        where a long heat up/cool down may limit the speed of polling        and/or increase the desired power consumption for a given        polling instance; and/or    -   High temperatures are not required, so safety may be provided in        environments that may be contaminated with explosive or        flammable vapors.

Indeed, photoelectrocatalytic sensing according to some embodiments ofthe invention may provide promising solutions for reducing oreliminating the need for thermal catalysis. However, because most metaloxide films are characterized by high band gaps in the UV range,photoelectrocatalytic sensing may exchange the potential problem of hightemperatures for the challenge of a low-cost, compact, ultravioletsource. Fortunately, compact commercially available UV LEDs are nowwidely available and can be integrated into a photoelectrocatalyticelectrode array as will be described in detail below. Thus, someembodiments of the invention can enable a low-cost, low-temperaturesolution for monitoring gaseous analytes within a wearable respirationmonitor.

FIG. 7 illustrates a photoelectrocatalytic sensor and real-time monitor700 of various fluid analytes 706 according to some embodiments of theinvention. As shown in FIG. 7, a photoelectrocatalytic layer 708 isintegrated with a support layer 707 and a contact layer 709 forcommunication with a signal processor 711. In some embodiments, thesupport layer 707 may comprise one or more conductive, insulating and/orsemiconductor layers that can perform various mechanical, electricaland/or optical functions in the photoelectrocatalytic sensor 700. Thecomposition of the support layer may vary based on the particularapplication and configuration of the photoelectrocatalytic sensor 700,and also may not be needed in other embodiments. In some embodiments,the contact layer 709 and the photoelectrocatalytic layer 708 form aphotoelectrocatalytic element 210, as was described in connection withFIGS. 2-4. The photoelectrocatalytic layer 708 is optically activated(“photo-excited” or “optically pumped”) by a semiconductor lightemitting source 703, also referred to herein simply as a “semiconductoroptical emitter.” Photo-excitation generates excited electrons and holescapable of catalyzing surface reactions between thephotoelectrocatalytic layer 708 and the analyte 706. The semiconductoroptical emitter 703 may have a guiding layer 704 to help guide opticalenergy 705 generated by the semiconductor optical emitter 703 towardsthe photoelectrocatalytic layer 708. The guiding layer may physicallycontact and/or be spaced apart from the support layer 707. Other opticalelements such as optical fibers, mirrors and/or lenses may be provided.The semiconductor optical emitter 703 can be located at any orientationthat allows optical energy 705 to fall on the photoelectrocatalyticlayer 708. Power and control electronics 702 (which may correspond tothe controller 330 of FIG. 3) may be integrated into a collectivehousing or package 701 with the semiconductor optical emitter 703, theguiding layer 704, the support layer 707, the photoelectrocatalyticlayer 708 and the contact layer 709. Signals generated in thephotoelectrocatalytic layer 708 through the interaction with one or moreanalytes 706 may be sent to a signal processor 711 and transmitted by asignal transmitter 712 to a signal analyzer 713 to generate desiredinformation from the photoelectrocatalytic sensing. The links betweenthe electronics 701, signal processor 711, signal transmitter 712 andsignal analyzer 713 may be wired and/or wireless. In some embodiments,an external monitor 710 may monitor photoelectrocatalytic-relatedactivity in or about the photoelectrocatalytic layer 708. An internalmonitor 714 also may be used in some embodiments, as will be describedin detail below.

The word “monitoring” refers to the act of measuring, quantifying,qualifying, estimating, calculating, interpolating, extrapolating,inferring, deducing, or any combination of these actions. Moregenerally, “monitoring” refers to a way of getting information via oneor more sensing elements. “Respiration monitoring” includes monitoringoxygen, carbon dioxide, nitrogen, carbon monoxide, volatile organicchemicals, and/or other respired gases and/or vapors. Generally, the useof the term “fluid” refers to liquids, gases, vapors, plasmas and/orparticles. For the sake of simplicity herein, a “vapor” is considered tobe a type of gas. However, embodiments of the present invention areapplicable to a variety of fluids and can be applied to these fluids bythose skilled in the art without deviation from the intent of thepresent invention. The terms “optically pumped” or “photo-excitation”refer to optical energy that is introduced into a material and theoptical energy interacts with the material such that excited states aregenerated in the material. Typically these excited states arephotoionized electron-hole pairs in a solid-state material, but othertypes of excited states may exist for various materials. The“photoelectrocatalytic” reaction refers to the process of generating acatalytic reaction at, near and/or within a material via optical pumpingof the material such that photo-excited states generated in the materialcan interact with one or more analytes at the surface of the material.The word “analyte” means any substance being identified and/or measuredby a sensor. Finally, the terms “semiconductor light emitting source,”“semiconductor optical source,” “semiconductor optical emitter” and/orvariants thereof may be used interchangeably herein.

The photoelectrocatalytic layer 708 may be any solid or reasonably solidmaterial capable of generating electrons and/or holes in response tooptical excitation 705. In some embodiments, the photoelectrocatalyticlayer 708 comprises a solid metal oxide and/or metal nitride film, suchas titanium dioxide, tin dioxide, aluminum oxide, gallium oxide, tinnitride, gallium nitride, aluminum nitride, indium nitride, alloys ofmetal oxides, alloys of metal nitrides, layers of metal oxides, orlayers of metal nitrides, metal oxy-nitrides, metal oxy-nitride alloys,metal nitride alloys (such as AlInGaN alloys), any combination of thesematerials and/or the like. These materials can be single crystalline,polycrystalline, amorphous, ceramic, polycrystalline, semimetallic,metallic and/or be composed of combinations of these morphologies. Forexample, the photoelectrocatalytic layer 708 may be composed of anamorphous metal nitride (such as an InGaAlN alloy) amorphous metaloxide, amorphous metal oxynitride and/or other amorphous material orcombination of materials. The contact layer 709, which in someembodiments is in direct physical contact with the photoelectrocatalyticlayer 708, can include any electrical contact. In some embodiments, theelectrical contact layer 709 comprises various conductive metals orpolymers patterned as electrodes on the surface of 708, such that one ormore regions of the photoelectrocatalytic material 708 are exposed tothe analyte 706. In some embodiments, the electrical contact layer 709may be transparent to the optical excitation energy. For example, indiumtin oxide, nickel oxide, nickel oxide gold, or thin (e.g., sub-nanometeror <5000 nm thick) metallic films can be sufficiently transparent to theexcitation light 705. In some embodiments, a transparent electricalcontact layer 709 may be used in embodiments where the semiconductoroptical emitter 703 is below the contact layer 709. The contact layermay also include a “reference electrode” (described below) and/or a“balancing electrode,” as described in FIGS. 30 and 31. The supportlayer 707 may be any solid or reasonably solid material capable ofsupporting the photoelectrocatalytic layer 708. In some embodiments, thesupport layer 707 serves as a substrate for the deposition of thephotoelectrocatalytic layer 708. Moreover, in some embodiments, asupport layer 707 may not be needed or the support layer 707 may be thephotoelectrocatalytic layer 708 itself. This substrate may be any solidfilm of sufficient smoothness to support an effectivephotoelectrocatalytic layer, such as a metallic substrate, asemiconductor substrate, a glass substrate, a polymer substrate, aplastic substrate and/or the like.

In embodiments shown in FIG. 7, the support layer 707 may be anoptically transparent material to allow optical energy 705 to passthrough the support layer 707 and be absorbed in thephotoelectrocatalytic layer 708. Thus, in these embodiments, thesubstrate may comprise glass, sapphire, quartz, transparent plastic, atransparent polymer and/or the like. However, in other embodiments, thesemiconductor optical emitter 703 is not located in the package 701 butrather is located outside the package. In these embodiments, thesemiconductor optical emitter 703 may be located on the bottom side ofthe photoelectrocatalytic layer 708 such that optical energy 705 reachesthe contact layer 709 first, followed by the photoelectrocatalytic layer708 and, lastly, the support 707. In such case, a transparent supportlayer 707 may not be needed. An example of this type of embodiment isshown in FIG. 8, and will be described in detail below. In yet otherembodiments, a semiconductor optical emitter 703 is not used, and thephotocatalysis is excited via ambient light, such as light from the sunreaching the photoelectrocatalytic layer 707.

The photoelectrocatalytic layer 708 and/or the contact layer 709 mayalso include selectivity (specificity) layers for preferential transportof the desired analyte 706 towards the photoelectrocatalytic layer 708.For example, layers 708 and/or 709 may include a membrane for selectivetransport of certain elements, chemicals, ions and/or particles towardsthe photoelectrocatalytic layer 708. Typically, these membranes are madeof selectively permeable polymers, lipid bilayers, biological ionchannels and/or artificial ion channels. In some embodiments, thesemembranes may include porous, polycrystalline, amorphous,nanostructured, or combinational layers for selective permeability totargeted fluids.

The semiconductor optical emitter 703 can be at least one semiconductoroptical source, such as a light-emitting diode (LED) and/or laser diode(LD). Note that specialized semiconductor LDs and LEDs such as organicsemiconductor light-emitting diodes (OLEDs), resonant cavity LEDs(RCLEDs), edge-emitting LEDs (EELEDs), and the like can also serve as asuitable semiconductor optical source 703. In other embodiments, amicroplasma ultraviolet source that is manufactured using semiconductorfabrication techniques, may also be used. Thus, as used herein, asemiconductor optical source can include any microelectronic opticalsource. In other embodiments, a non-semiconductor optical emitter, suchas a laser, lamp, the sun and/or another natural optical source, may beused.

In some embodiments, the semiconductor light emitting source is acompact point-source capable of being packaged together with othercomponents in an integrated package 701. In other embodiments, thesemiconductor light emitting source may be an LED or LD integrated intoa monolithic package 701 with the photoelectrocatalytic layer 708 andassociated layers 704, 707, and 709. A guiding layer 704 may be incontact with the semiconductor optical emitter 703 for the purposes ofguiding optical energy 705 towards the photoelectrocatalytic layer 708.The guiding layer 704 may be any solid or reasonably solid film capableof guiding, filtering, or directing light. In some embodiments, theguiding layer 704 has a higher refractive index than air. In otherembodiments, the guiding layer includes an optical lens, mirror, opticalwaveguide, fiber optic cable, an optical filter, combinations of theseand/or the like.

Since it may be desirable for the guiding layer 704 to be at leastpartially transparent to the excitation light 705, quartz, sapphire,metal nitrides, or fluoropolymers, or other UV-transparent materials maybe used for guiding UV light. For the case of photo-excitation using anIR-light source, this material may comprise zinc selenide, amorphousmaterials transmitting IR, or other IR-transparent media. Standard glassand topaz may be used for guiding visible light, but UV-transparentmaterials may also be effective for guiding visible light. Because anLED source may image the geometry of the optically emitting LED surfaceonto a receiving surface, it may be desirable to diffuse the LED beampattern in order to uniformly distribute the light over thephotoelectrocatalytic layer 708. In such embodiments, the guiding layer704 may be intentionally roughened to scatter light 705 for a moreuniform optical excitation of the photoelectrocatalytic layer 708. Avariety of guiding layer structures also can be implemented fordiffusing light without roughening the surface. For example, intentionaldefects added to the guiding layer, such as various dopants and/orcrystalline defects, can scatter light. A variety of scattering surfacesand structures for diffusive guiding layers are well known to thoseskilled in the art. In other embodiments, the guiding layer can becomposed of one or more layers for collecting light, guiding light,scattering light and/or diffusing light. In some embodiments, theguiding layer 704 may not be used, and the layer 704 can be absent fromthe structure. As described earlier, the semiconductor optical emitter703 may be on either side of the photoelectrocatalytic film 708, butFIG. 7 shows the optical emitter 703 directing optical energy 705towards the support layer 707 first, followed by thephotoelectrocatalytic layer 708.

The electronics (controller) 702 may be any combination of electricalcomponents that can be used for powering, controlling, amplifying,signal conditioning, and/or regulating the semiconductor optical emitter703 and/or the photoelectrocatalytic electrodes of the contact layer709. In some embodiments, the electrical components include a powersource (in some embodiments a battery), a modulating source, a powerregulator, a signal processor, an analog-to-digital converter, a signalamplifier and/or the like, all together within the integrated package701. Electrical circuits supporting a sensor are well known to thoseskilled in the art, and need not be described in detail herein.

The analyte 706 may be any fluid (as defined previously), but in someembodiments is a gas or particle, or any combination of gases orparticles, capable of interacting with the photoelectrocatalytic layer708, the contact layer 709, or a combination of these layers. In someembodiments, the analyte 706 comprises ionizable gases and vapors, suchas oxygen, carbon dioxide, nitrogen, carbon monoxide, ozone, humidity,volatile organic chemicals, aromatics, pollutants, polycyclic aromatichydrocarbons (PAHs) and/or the like. The analyte 706 can also includevarious other chemicals and particles including airborne pollutants,biological particles (such as allergens, fungi, bacteria, viruses,organic material, etc.), hydrocarbons, soot particles and/or the like.In some embodiments, the photoelectrocatalytic sensor system 700 isemployed as a respiration monitor, for detecting, qualifying, and/orquantifying gases, vapors and/or particles in the breath.

The signal processor 711 may be used to process or pre-process raw orpreprocessed signals from the photoelectrocatalytic element(s). Thisprocessed information may then be sent to a signal transmitter 712 forthe transmission of processed data to a signal analyzer 713. In someembodiments, the signal transmitter transmits data wirelessly to one ormore signal analyzers 713. Signal processors and signal transmitters arewell known by those skilled in the art, and various types of signalprocessing electronics and electrical transmission electronics caninclude commercial off-the-shelf parts. The signal analyzer 713 may beany device capable of computing, such as a laptop computer, a personaldigital assistant (PDA), a desktop computer, a cell phone, a smartphone,a calculator and/or the like. In some embodiments, the signal processor711 and the signal transmitter 712 are all integrated in the package701. In other embodiments, the signal analyzer 713 is also integrated inthe package 701.

Some embodiments may also include an external monitor 710. The externalmonitor 710 may be any monitor capable of analyzingphotoelectrocatalytic related activity in and/or about thephotoelectrocatalytic layer 708, the contact layer 709 or thecombination of these layers. In some embodiments, the external monitor710 measures electromagnetic, mechanical, acoustic and/or thermal energyat the surface, and/or other changes at the surface of thephotoelectrocatalytic layer 708. In some embodiments, the externalmonitor 710 may eliminate the need for the contact layer 709 and/oreliminate the need for electrical connections or wires to the contactlayer 709. For example, this can be achieved by inductive or opticalcoupling to the contact layer 709 or photoelectrocatalytic layer 708. Insome embodiments, the external monitor 710 is not included in thephotoelectrocatalytic sensor 700 at all. In some embodiments, the roleof the external monitor can be replaced with an internal monitor 714,housed inside the package 701, as will be described below.

Some embodiments of the invention can provide the photoelectrocatalyticsensor itself, which may be represented by the package 701, and at leastsome elements therein. The elements outside this package may also beadded in some embodiments to provide additional functionality and/orperformance. The package 701 may be as simple as a standard TO-5 packageor substantially more intricate, such as a stainless steel vacuumpackage. The packaging materials may include virtually any solid,reasonably inert material, such as metal, plastic, rubber, ceramicand/or the like.

The photoelectrocatalytic layer 708, the support layer 707, and thecontact layer 709 may form the photoelectrocatalytic element in someembodiments. In other embodiments, the photoelectrocatalytic element mayconsist of the photoelectrocatalytic layer 708 and, in otherembodiments, the photoelectrocatalytic element may comprise one or moreother structure(s) in addition to the photoelectrocatalytic layer 708.The photoelectrocatalytic element may also be referred to herein as a“photoelectrocatalytic electrode” or simply as an “electrode.” Theseelectrodes can be one or more photoelectrocatalytic electrodes composedof one or more photoelectrocatalytic films. For example, theseelectrodes can be arrayed as illustrated in FIG. 9, as described in moredetail below.

Other embodiments of a photoelectrocatalytic sensor 800 are shown inFIG. 8. In these embodiments, the semiconductor optical emitter 803 isat least one LED or LD, and it is positioned outside of the sensorpackage 801. The semiconductor light emitting source 803 shines light805 onto an array of photoelectrocatalytic films 808 to facilitatephotocatalysis of one or more analytes 809 at and/or near the array ofphotoelectrocatalytic films. The patterned metal contacts 806 arecombined with the photoelectrocatalytic films 808 to formphotoelectrocatalytic electrode arrays.

It should be noted that the photoelectrocatalytic films 808 and contacts806 need not be the same for each element of the array. Specifically,multiple photoelectrocatalytic films 808 and/or metal contacts 806 canbe used to distinguish between multiple fluid analytes in mixtures ofgases, vapors and/or particles. Certain analytes can be qualified andquantified more accurately when compared with a reference analyte of aknown concentration, so that multiple photoelectrocatalytic films 808and metal contacts 806 may be provided. Further, the uniqueabsorption/desorption kinetics and photoelectrocatalytic response ofeach analyte 809 with respect to each photoelectrocatalytic layer 808,though sometimes subtle, can be used to differentiate between individualanalytes in a mixture of analytes, in some embodiments.

A selective membrane 804 may be used in some embodiments to reduce orprevent at least some unwanted matter or energy from approaching thephotoelectrocatalytic electrodes while selectively passing one or moreanalytes. This layer may not be used in some embodiments because thephotoelectrocatalytic films 808 may be chosen to be preferentiallysensitive to one type of analyte and/or for other reasons. A protectivelayer 807 may be used to protect parts of the metal contacts 806 and/orphotoelectrocatalytic films 808. The protective layer 807 can reduce orprevent unwanted reactions near the metal/photoelectrocatalytic filminterface and may also serve as electrical and/or electrochemicalpassivation. A solid substrate 802 serves as a support layer.

The photoelectrocatalytic layers 808 and contacts 806 can be made of thematerials described previously for 708 and 709. In some embodiments, thephotoelectrocatalytic films 808 are thin metal oxide films and thecontacts are metals and/or electrically conducting polymers. Althoughfor simplicity only two photoelectrocatalytic electrodes are shown inFIG. 8, it is to be understood that any number of photoelectrocatalyticlayers and/or contacts can be used to facilitate selectivity inmeasuring various analytes and/or for other purposes. In someembodiments, the selective membranes 804 can comprise polymers,dielectrics, ion-selective electrode materials and/or particle sizefilters. These materials are commercially available and well known tothose skilled in the art. The protective layer 807 may comprise aninsulating dielectric film such as SiO₂, SiN, polymers, tape and/or thelike. Although layer 807 is shown to partially cover the contact layers806 in FIG. 8, the protective layer 807 may cover any area, or theentire area, of the contact layers 806 and/or photoelectrocatalyticlayers 808. The package 801 itself may be compatible with TO-5,Bergquist and/or similar standardized packaging arrangements.

A top-view of the photoelectrocatalytic sensor 800 is shown as 900 inFIG. 9. Only the photoelectrocatalytic films 908, the contact layers906, and the support layer 902 are shown for simplicity. Note thatexemplary rectangular geometries are drawn for the photocatalyticelectrodes, but other shapes, such as circles, squares, triangles and/orother shapes may be used in other embodiments. Moreover, althoughcontact layers 906 are shown on top of the photoelectrocatalytic films908, these contacts can be placed at the edges, the center, the cornersand/or virtually anywhere along the photoelectrocatalytic film surfacein other embodiments. Enough space should be left between the contactsto allow detection of absorbed analytes near the surface of thephotoelectrocatalytic films 908. Also, although only two contacts perphotocatalytic film are shown, three or more contacts may also beemployed in other embodiments.

Embodiments of a photoelectrocatalytic sensor 1000 integrated with atleast one underlying LED or LD are shown (in side-view) in FIG. 10. Thesemiconductor light emitting source 1003 generates optical energy 1005directed towards one or more photoelectrocatalytic elements 1008 in anarray. In FIG. 10, the photocatalytic elements 1008 include both thephotoelectrocatalytic films and their respective electrical contacts.The signal from each photoelectrocatalytic element 1008 is read throughmultiple contact pins 1006 at the edge of the device. These contact pinsmake contact with circuitry 1004 for individual communication with eachphotoelectrocatalytic electrode. A submount layer 1009 is used tosupport heat extraction in the optical emitter 1003 and/or for physicalsupport. The submount layer 1009 may also include metal contacts fordelivering electrical power to the LED or LD. The submount layer 1009may be connected with a secondary mounting layer 1010 for additionalthermal extraction, mechanical support and/or electrical power.Electronics 1011 may be integrated within the sensor 1000. The entiremonolithic photoelectrocatalytic sensor may be enclosed in a package1001. The analyte 1007 is shown in direct contact with thephotoelectrocatalytic electrodes, without a clearly distinguishedselective transport layer 804 protecting the films. Although such a filmcan be incorporated as in FIG. 8 or selectively deposited on eachphotoelectrocatalytic electrode 1008, this film may not be incorporatedfor selective sensing of various analytes in other embodiments.

In FIG. 10, the materials that may be used may be the same as FIG. 8 andFIG. 9. However, it should be noted that the submount layer 1009 andsecondary mounting layer 1010 may be used in embodiments of FIG. 10, asthe semiconductor optical source 1003 is part of the monolithicphotoelectrocatalytic array 1000. These layers, in some embodiments, canbe interchangeable or, in some cases, the submount 1009 need not beused. For example, the bond pad metallization of a flip-chip LED 1003can be lined up with metallization patterned directly on a Bergquistthermal package—including a circuit layer, dielectric material, andmetallic base material—such that a submount layer need not be used. Inmany cases, the submount 1009 and secondary mounting 1010 can beintegrated into one unit to reduce the thermal resistance between thetwo layers. The electronics 1011 may include powering, amplifying,signal conditioning, signal transmitting, signal processing and/orsimilar electronics. These electronics may be located near the bottom ofthe sensor 1000, as shown in FIG. 8, but other locations may be used. Insome embodiments, the electronics are not included in the sensor 1000but rather are bifurcated from the sensor altogether. Moreover, as withall the embodiments herein, multiple optical sources 1009 may beintegrated into the sensor 1000. In some embodiments, these opticalsources may be specifically aligned to one or more PEC electrode(s)1008. In some embodiments, at least one of these optical sources maygenerate a different optical wavelength than another optical source.

As was described above, other embodiments of the invention providemethods of monitoring fluids using a photoelectrocatalytic sensor. Thesefluids may comprise gases and vapors from exhaled breath, airborne andwaterborne contaminants and/or industrial pollution. Monitoring fluidsfrom each case may involve different sensor designs. For example, arespiration monitor may be mounted on a portable headpiece or amouthpiece. In some embodiments, a standardized portable phoneheadset—including a headpiece, an earpiece, and mouthpiece—incorporatesone or more photoelectrocatalytic sensors in the mouthpiece. Such asensor can facilitate oximetry, capnometry and/or health diagnoses. Insome embodiments, the PEC sensors can be mounted in a headpiece,headset, cellular phone, PDA, earpiece, or the like without the need fora mouthpiece.

A photoelectrocatalytic sensor used as an environmental monitor may belocated virtually anywhere. For example, an industrial pollution monitormay be located at or near one or more exhaust ports. In some cases,pollution may come from a motor vehicle, in which case aphotoelectrocatalytic sensor may be at or near the exhaust of thevehicle. In some embodiments, the sensor may be mounted on a wall, on atripod, or within another piece of equipment such as a fire alarm, gasalarm, or other unit. For example, sensor modules may be placedthroughout a building Heating, Ventilating, Air-Conditioning (HVAC)system in order to pinpoint air contamination and/or more efficientlydirect exhaust. In some embodiments, a portable environmental monitorcomprising a photoelectrocatalytic sensor can be incorporated in aportable wireless instrument such as an earpiece, headset, cell phone,PDA and/or the like. Because photoelectrocatalytic sensors need not usea heater filament to initiate the catalytic reaction, these sensors canwork well for wearable, human-portable sensors as well asexplosion-proof sensors.

It should be noted that the photoelectrocatalytic layers 708 do not needto be layers that respond only to UV light to initiate photocatalysis.For example, indium oxynitride can have a band gap ranging from themid-IR to deep-UV. Thus, employing a photoelectrocatalytic layer ofindium oxynitride in a photoelectrocatalytic sensor can facilitatephotocatalysis via IR or visible light as well as UV light, depending onthe stoichiometry of the indium oxynitride alloy. Thus, an integratedphotoelectrocatalytic sensor of indium oxynitride can incorporate an LEDin the visible or IR wavelengths as well as the UV. Moreover, nuclearradiation or X-rays can also be used to ionize the surface of aphotoelectrocatalytic sensor in some embodiments.

It should also be noted that desired properties of thephotoelectrocatalytic layers according to other embodiments of theinvention can be realized by nanoengineering the surface layers and/orbulk layers. For example, adding nanoscale features can improve thesensitivity of the photoelectrocatalytic film to gaseous species.Furthermore, nanoengineering the surface and/or bulk layers with regularor irregular nanoscale features can improve the sensitivity to specificanalytes and thus can provide a selective, or reasonably selective, gassensing layer that can be specific to one particular species or class ofspecies. Additionally, the nanostructure of the surface of thephotoelectrocatalytic film can affect the wavelengths used forphotocatalysis. For example, through the quantum-size-effect, theeffective band gap of the surface can be increased or decreased bydecreasing or increasing (respectively) the size of nanoscale featuresin the photoelectrocatalytic film. A variety of nanoscale features canbe applied, such as nanorods, nanowires, nanospheres, nanocubes,nanodots, nanowhiskers, nanocoils, nanoribbons, nanodoughnuts,nanopyramids, nanodomes, nanotemples, nanosteps, nanospirals,nanogratings, nanotrenches, nanocapacitors, nanodisks, nanohexagons,nanopentagons, buckyballs, nanodrops, nanoporous films, nanocubes,disordered nanostructures, nanoballons, and the like. This includesnanoscale removal of material, such as in the case of making thematerial porous via photon-assisted etching techniques.

In other embodiments, explicit nanoengineering need not be used to formpractical nanostructures in metal oxide films. For example, in manycases these nanostructures can form naturally via spontaneous orderingduring the thermodynamics of film growth or film deposition. In othercases, nanostructured features may be directly engineered throughphotolithographic techniques, block copolymer techniques, selectivedeposition techniques, thermodynamic methods and/or the like.

Other embodiments of the invention can provide analyte monitors capableof identifying and quantifying multiple species in real time. Someembodiments can combine solid state optical emitters withphotoelectrocatalytic electrodes, enabling a low-cost, low-power,low-profile, portable/wearable and/or real-time analyte monitor. In someembodiments, the analyte monitor can operate as a personal environmentalassessment monitor for monitoring exposure to VOCs, PAHs, ozone, and/orother airborne pollutants and/or toxins. Some embodiments can provide aportable monitor for volatile organic compounds.

VOC sensors according to some embodiments of the invention can operateby monitoring the electrochemical response to the known photocatalyticeffect of titania, zirconia, and various other metal oxides in thepresence of different gases and vapors. In some embodiments, ultravioletlight generates electron-hole pairs (and quasi Fermi levels) in themetal oxide film, and these electrons/holes are consumed to catalyzeairborne species at the metal oxide surface.

Conventionally, the photocatalytic effect of titania has been appliedtowards the biological sterilization of air and water. Inphotoelectrocatalytic sensing according to some embodiments of theinvention, the photocatalyzed byproducts of gas and vapor oxidation arenot simply discarded but rather are monitored electrically between twoor more conductive contacts as illustrated schematically by thevoltmeters in FIG. 11. This can provide a convenient tool for monitoringthe presence and/or relative intensity of adsorbed gases and vapors.Depending on the stoichiometry, porosity, morphology and/or type of themetal oxide film, different gases can be selectively adsorbed anddetected. Metal nitrides, metal arsenides, and other semiconductormaterials can also make excellent photocatalysts. Moreover, when metaloxides are referenced, it should be understood that other solid-statephotocatalysts can also be used in other embodiments of the invention.In some embodiments of the invention, analyte selectivity may beachieved by monitoring the transient electrical response that is uniqueto each ionized species. In other embodiments, analyte selectivity maybe achieved by monitoring and comparing the electrical response frommultiple electrodes having preferentially selective catalysts, asillustrated in FIG. 11. Other techniques for achieving analyteselectivity will be described in detail below.

For example, as shown in FIG. 11, under the UV excitation of titania ina humid ambient, photogenerated holes convert adsorbed water vapor intoH+ and hydroxyl radicals (.OH) that can oxidize volatile organiccompounds (“R” in FIG. 11) at the titania surface. Similarly, oxygenadsorbed under UV excitation generates superoxide anions which alsooxidize VOCs. If the photoelectrocatalytic metal oxide film isfabricated as an electrode, the electrical response to adsorbed,photocatalyzed analyte can be directly related to the analyteconcentration.

Photocatalytic sensing according to various embodiments of the presentinvention may be sharply contrasted with conventional thermocatalyticsensing, particularly in the context of VOC monitoring via metal oxidesensors. In photoelectrocatalytic sensing, photogenerated electron-holepairs are primarily responsible for VOC catalysis, such that certainspecies can be chemically reduced (by the transfer of electrons from themetal oxide) or oxidized (by the transfer of holes from the metaloxide). In contrast, for thermocatalytic sensing, thermal energy isprimarily responsible for VOC catalysis. Namely, thermal energy in themetal oxide sensor can raise the Fermi energy such that electrons can beinjected into the gaseous species, reducing the vapor at the surface,thus spawning the generation of surface radicals which are then adsorbedat the metal oxide surface. For the case of VOCs on thermally heatedSnO₂, these adsorbed radicals behave as donors, providing free carriersfor electrical conduction.

Characteristic of both thermocatalysis and photocatalysis, theconductivity response from adsorbed radicals can be relatively slow, andmay take several seconds for equilibration. However, for the case ofphotocatalysis there can be an additional conductivity responseassociated with the generation of electron-hole pairs. Normally,photogenerated electron-hole pairs recombine within nanoseconds ofoptical excitation. However, as photogenerated holes are preferentiallyconsumed during the photocatalysis of oxidizable VOCs, an accumulationof negative charge can build up at the metal oxide surface as will bedescribed below in connection with FIG. 14. This negative chargeaccumulation can be monitored both conductively (as photocurrent) andcapacitively. Moreover, if the UV photo-excitation is modulated on andoff, these free electrons will accumulate and recombine accordingly suchthat the time-dependent concentration of these unpaired electrons can bedistinguished from noise and clutter. Because this surface chargeresponse can be several times faster than the adsorbed radical chargeresponse, these two independent PEC responses can be distinguished inthe same circuit or in different circuits, which can provide selectivedifferentiation between various VOCs via a single sensor according tosome embodiments of the invention. Furthermore, the sensitivity of thePEC response can be controlled independently of temperature bycontrolling the wavelength and/or intensity of photo-excitation.Additionally, because PEC analyte sensitivity need not use elevatedtemperatures, where solid-state devices experience elevated electricalnoise, the signal-to-noise ratio of PEC sensors can be much greater thanthat of thermocatalytic sensors.

Additionally, monitoring the time-dependent PEC response of a singlecatalyst can also provide enhanced discrimination between VOCs. Thistemporal response may be limited primarily by the recombination rate ofexcess carriers (holes or electrons) at the metal oxide surface and/orthe adsorption/desorption rate of the gaseous species within the metaloxide film. The latter can be controlled to some degree by the thicknessand/or morphology of the film. By monitoring signal rise-time in asingle PEC sensor, outstanding discrimination between various VOCs canbe realized through principle component analysis.

Other techniques for achieving VOC specificity can implement an array ofmultiple catalytic layers having selective (or, in some embodiments,quasi-selective) photoelectrocatalytic properties. Various catalysts canbe characterized by unique surface interaction properties and specificchemical potentials (Fermi energies). In particular, the application ofgallium, indium and/or zinc oxide photocatalysts in PEC sensing canprovide a stable, robust PEC surface for the selective photocatalysis ofvarious gases, in some embodiments of the invention.

Indeed, photoelectrocatalytic sensing according to some embodiments ofthe invention may provide a promising solution for reducing oreliminating the need for thermal catalysis. However, because most metaloxide films are characterized by high band gaps in the UV range,photoelectrocatalytic sensing may exchange the problem of hightemperatures for the challenge of a low-cost, compact, ultravioletsource. Fortunately, compact commercially available UV LEDs are nowwidely available and can be integrated with aforementioned metal oxideelectrodes, providing a packaged PEC VOC sensor.

Sensing using the photoelectrocatalytic effect according to someembodiments of the present invention has been demonstrated using amodified commercially available thermocatalytic tin oxide VOC sensor.This configuration was used solely for ease of experimentation, andembodiments of the invention are not dependent on the thermocatalyticsensor technology. Thus, the following experimental example is providedfor illustrative purposes and shall not be regarded as limiting theinvention. As illustrated in FIG. 13A, in this experiment, a 280 nm UVLED was aligned head-on with the filament-heated tin oxide sensor, withthe entire setup covered by a large glass beaker. The tin oxide VOCsensor includes a simple tin oxide ceramic sintered onto an aluminacylinder surrounding a heating coil. The heating coil was disconnectedin this experiment such that conventional thermocatalysis would not takeplace. Instead, about 100 μW of 280 nm UV light from a UV LED wasfocused directly onto the unheated tin oxide sensor, and thephotoelectrocatalytic (PEC) conductivity response to varying ethanolconcentration was measured using a simple biasing circuit (inset of FIG.13B) with an oscilloscope used to monitor the electrical response of thesensor. The VOC concentration was calibrated using a second VOC monitorsituated inside the glass beaker (not shown in FIG. 13A), operated inthermocatalytic mode.

Ethanol vapor was introduced into the covered setup by placing a smallpetri dish of liquid ethanol underneath the glass beaker at roomtemperature. A nonlinear increase in the PEC response with increasingethanol concentration was observed (FIG. 13B) under steady UV light.This type of logarithmic response is characteristic of catalysis and maybe expected for this sensing configuration. In the absence of UV light,the PEC signal decayed to zero, regardless of the ethanol concentration,over the course of a few seconds. Additionally, a differential responsewas never detected in the absence of UV light. A stronger PEC responsecould be observed by activating the heater filament at low current suchthat the tin oxide sensor could reach a temperature slightly higher thanroom temperature (e.g., 320K). This reduced the baseline conductivity ofthe tin oxide sensor to more easily measurable values and encourageddesorption of the ethanol from the service between test runs.

To verify that localized optical heating was not responsible for theconductivity response, the UV LED was replaced with a bright (about 10mW) white LED and a 6 mW red laser pointer, neither of which emitsignificant UV light. In each case, a PEC response of FIG. 13B was notobserved, regardless of the ethanol concentration. This example providesa high level of confidence that a UV-induced photoelectrocatalyticreaction is responsible for the observed ethanol sensing.

FIGS. 13C and 13D provide other experimental examples of sensing usingthe photoelectrocatalytic effect according to some embodiments of thepresent invention. The following experimental examples also are providedfor illustrative purposes, and shall not be regarded as limiting theinvention.

As illustrated in FIG. 13C, photoelectrocatalytic testing of ethanol,methanol, acetone and water vapor were performed using the configurationshown. Ozone testing was executed using a 280 nm UV LED excitationsource, and ozone was generated using an optically shielded deuteriumlamp. Vapor testing was baselined using a tin oxide heated-filamentalcohol sensor marketed by Futurlec. The Futurlec sensor is known torespond to various volatile organic vapors with virtually identicalsensitivity.

As illustrated in FIG. 13D, under photoelectrocatalytic excitation, theInGaZnO MSM showed a strong sensitivity to acetone and ethanol vapors,and a modest sensitivity to methanol vapor, but no marked sensitivity towater vapor or ozone. In contrast, the ZnO MSM was sensitive to watervapor and ozone, but showed no marked sensitivity to any of the volatileorganic vapors. Because the concentration of water vapor and ozone couldnot be baselined with the Futurlec sensor, only a binary on/off responsecould be observed.

Without wishing to be bound by any theory of operation, it is theorizedthat the chemical selectivity of each metal oxide film may be due to themicro- and/or nano-structure of the InGaZnO in comparison to the ZnOfilm. The ZnO is mostly polycrystalline, whereas the InGaZnO film iscomposed of polycrystalline components and amorphous components. Thus,surface adhesion, adsorption/desorption and/or electrochemicalproperties may be different for each film in the presence of differentvapors.

FIG. 12 illustrates a PEC VOC monitor according to some embodiments ofthe invention, integrated into a Bluetooth module. The VOC sensorelement includes a UV LED integrated with a PEC electrode array,packaged together in a milled TO-header. The PEC electrodes areconnected to the TO-header through, for example, gold wire bonds. Themultiple PEC electrodes can enhance the specificity of the VOC monitor.In embodiments of FIG. 12, each PEC electrode is composed ofinterdigitated electrodes, though a variety of other electrodeconfigurations may be used. The interdigitated configuration can allowconductivity, photocurrent, capacitance, and impedance to be monitoredin the same device. The packaged VOC sensor element can be integratedinto a compact Bluetooth module for wireless communications. However, avariety of wired or wireless modules other than Bluetooth, such asZigBee, IEEE 802.11b,g, or the like can be used. Regardless of theprotocol used, the compact, portable, low-power module of FIG. 12 canprovide wearable VOC monitoring.

Without wishing to be bound by any theory of operation, the fundamentalphysics and electrochemistry of the PEC effect for semiconductorcatalysts, such as metal oxide catalysts, is summarized pictorially inFIGS. 14-16. As will be described below, FIG. 16 describes a method ofPEC sensing where sub-band-gap visible or IR radiation triggers PECsensing, without the need for UV wavelengths and UV optical sources.

FIGS. 14A-14C illustrate the electrochemistry for the oxidation of ananalyte near a solid-state catalyst via the photoelectrocatalyticeffect. In this case, the chemical potential (or Fermi potential) of theanalyte (E_(F)-analyte) is lower than that of the solid-state catalyst(E_(F)-catalyst) (FIG. 14A). At the analyte-catalyst interface, theestablishment of electrochemical equilibrium results in an electricfield at the interface, pointing towards the analyte and away from thecatalyst (FIG. 14B). At the microscopic level, this field is caused bythe redistribution of charge at the analyte-catalyst interface due to adifferential in charge affinity for the differing materials. Thephoto-excitation of the catalyst layer (at photon energies at or abovethe band gap energy of the catalyst) generates quasi Fermi levels (shownas dotted lines) and electron-hole pairs as shown in FIG. 14C. Theelectric field at the analyte-catalyst interface ejects holes from thecatalyst layer (pulling electrons from the analyte) thereby oxidizinganalyte near the surface. As the reactive byproducts leave, a netnegative charge resides at the catalyst surface, and this negativecharge can be measured electrically via surface electrodes.

The process in FIGS. 15A-15C is somewhat reversed, as the chemicalpotential of the analyte is higher than that of the catalyst (FIG. 15A).Thus, under chemical equilibrium, the electric field at the interfacepoints away from the analyte and towards the catalyst (FIG. 15B). Underphoto-excitation, free electrons are injected into the analyte, reducingthe analyte, leaving a net positive charge at the catalyst surface (FIG.15C). This polarity of the charge response can allow a PEC VOC monitorto differentiate between oxidizable and reducible volatile species atthe PEC surface.

FIGS. 16A-16C illustrate sensing reducible analyte species via the PECeffect. In these embodiments, the catalyst is doped with deep-levelintentional impurities (E_(impurity)), such as rare-earth dopants(erbium, europium, promethium and/or the like) (FIGS. 16A and 16B).Thus, sub-band-gap photo-excitation can trigger the injection ofelectrons into the analyte, reducing the analyte, and thus providing anet positive charge at the surface for electrical sensing (FIG. 16). Inthe presence of oxidizable analyte, the net surface charge will benegative. Embodiments of FIGS. 16A-16C can allow oxidizable andreducible analyte species to be monitored with visible or infrared lightas opposed to UV light. Because visible and infrared LEDs may be moreefficient, reliable, affordable and/or readily available than UV LEDs,these embodiments may be used in some portable VOC sensing applications.

From the physics of operation outlined in FIGS. 14, 15, and 16, onetechnique for potentially increasing the sensitivity of the PEC effectin conductive detection mode is to run higher than nominal currentsthrough the electrodes to sweep excess charge carriers before carrierrecombination. This type of configuration may be used for PEC sensingwhere current is passed between at least one electrode.

Any of the embodiments described herein may be used for environmentalmonitoring, such as VOC monitoring, in addition to respirationmonitoring. “VOC monitoring” includes monitoring benzene, ethanol,hexane, aromatics, ketones, aldehydes and/or other volatile compoundsthat can be gaseous at room temperature. In some embodiments, the PECsensor is employed as an environmental monitor, for detecting,qualifying and/or quantifying gases, vapors, and particles in air—forexample, a monitor for VOCs, PAHs, ozone, carbon monoxide, NO and/or thelike. In some embodiments, this environmental monitor PEC sensor systemmay be compact, wearable and/or integrated onto a telemetric module,such as a Bluetooth headset module, for monitoring one's personalenvironment and transmitting this information to a cell phone, personaldigital assistant (PDA), computer, database, or the like (FIG. 12).

In some embodiments, an external monitor 710 and/or internal monitor 714of FIG. 7 can be used to monitor other forms of energy duringphotocatalysis. For example, a gas or vapor 706 which breaks down intobyproducts under photocatalysis from UV light 705 may result inbyproducts which absorb, reflect, or fluoresce when exposed to UV light.This can be monitored by a photodiode 714 housed inside the package 701.A specific example of this type of device 2700 is shown in FIG. 27.

Referring to FIG. 27, in these embodiments, the milled TO-headercontains a mounting bracket holding two photodiodes on either side ofthe UV LED. As UV light from the UV LED induces photocatalysis in thePEC electrodes, the byproducts of the vapor analyte fluoresce, and thisenergy can be used to potentially increase the specificity of the PECsensor to the analyte species measured. As a particular example, if theliquid or vapor analyte contains molecules of nicotinamide adeninedinucleotide (NAD+), the UV-induced PEC effect can reduce at least someof the NAD+ into NADH, which is a fluophore emitting blue light under UVexcitation. In this way, both the electrical response of the PEC effectand the fluorescence response of the PEC byproducts can be measured inthe same device, which can provide enhanced specificity for identifyingone reducible analyte from another.

Combined PEC+ fluorescence device 2700 of FIG. 27 can be used to allowimproved specificity, concentration accuracy and/or sensitivity ofmeasuring volatile analyte species that are themselves both volatile andfluorescent. For example, a variety of plant oils are volatile under UVphoto-excitation as well as fluorescent under UV excitation. A combineddevice according to some embodiments of the invention can be used toallow improved specificity of sensing such species of analyte. Otheroptical properties of the PEC film 708 under optical excitation 705 inthe presence of analyte 706, such as reflection, diffraction,absorption, plasmon interaction, phonon interaction, and the like, canalso be monitored during photocatalysis using the external 710 orinternal monitor 714, and this can provide further sensor specificity toa particular type of analyte.

For example, the PEC effect can be used in conjunction with surfaceplasmon resonance (SPR) to monitor analyte at the PEC layer 708. In suchembodiments, electromagnetic probing energy, such as optical probingenergy, hitting the PEC layer is altered by plasmon generation at thePEC layer 708 in response to the combination of analyte 706 andphoto-excitation 705 at the PEC layer 708 surface. The optical probingenergy can be altered by a change in reflection, diffraction,transmission, absorption, polarization, wavelength, phase, intensityand/or the like in response to plasmon interactions at the PEC layer708. SPR plasmon generation is usually generated by laser excitation ata surface, and this may be incompatible with a portable SPR monitor.However, in the PEC/SPR embodiments just described, the plasmons can begenerated by an integrated LED, such as that of FIG. 17 or the like, aswill be described in detail below. Similarly, the optical probe energyand optical probe energy monitoring can be accomplished with theexcitation energy itself and parallel photodiodes, as shown in FIG. 27.In these embodiments, the excitation energy may not only be responsiblefor exciting the plasmons but also the optical probe energy—theexcitation energy is altered by the plasmon/optical interaction, andthis altered optical energy may be monitored by the photodiodes. Othertypes of integrated optical detectors can replace the photodiodes, withpolarization detectors, diffraction gratings and/or the like. It shouldbe understood that various secondary detectors (710 or 714) and detectorconfigurations may be used to monitor analyte during PEC sensing, andFIG. 27 is merely one example.

Other embodiments of a photoelectrocatalytic sensor 1700 integrated withat least one underlying LED or LD, is shown in side-view in FIG. 17. Thesemiconductor light emitting source 1703 generates optical energy 1705directed towards one or more photoelectrocatalytic elements 1708 in anarray. In FIG. 17, the photocatalytic elements include both thephotoelectrocatalytic films and their respective electrical contacts.

It should be noted that the photoelectrocatalytic films and contactsneed not be the same for each element of the array. Specifically,multiple photocatalysts and metal contacts can be used for the purposeof distinguishing between multiple fluid analytes in mixtures of gases,vapors and/or particles. Certain analytes can be qualified andquantified more accurately when compared with a reference analyte of aknown concentration, so that multiple photoelectrocatalytic films andmetal contacts may be provided. Further, the uniqueabsorption/desorption kinetics, surface interaction physics and/orphotoelectrocatalytic response of each analyte 1707 with respect to eachphotoelectrocatalytic electrode 1708, though sometimes subtle, can beused to differentiate between individual analytes in a mixture ofanalytes. The signal from each photoelectrocatalytic electrode 1708 maybe read through multiple contact pins 1706 at the edge of the device.These contact pins make electrical contact with the array circuitry 1704for individual communication with each photoelectrocatalytic electrode.In some embodiments, the multiple electrical contacts are made through,for example, gold wires bonded between the PEC electrodes 1708 and thecontact pins 1706. A submount layer 1709 is used to support heatextraction in the optical emitter 1703 and for physical support. Thesubmount layer 1709 may also include electrically conductive contactsfor delivering electrical power to the LED or LD. The submount layer1709 may be connected with a secondary mounting layer 1710 foradditional thermal extraction, mechanical support and/or electricalpower. Electronics 1711 may be integrated with the sensor 1700. Theentire monolithic photoelectrocatalytic sensor may be enclosed in apackage 1701. The analyte 1707 is shown in direct contact with thephotoelectrocatalytic elements 1708, without a secondary layer toprotect the electrical contacts, such as a dielectric layer, polymerlayer, or the like. Although such a protective layer can be incorporatedat or near the surface of the PEC electrodes 1708, or selectivelydeposited on each photoelectrocatalytic electrode, this layer may not beneeded for selective sensing of various analytes in many environments.

The photoelectrocatalytic elements 1708, composed primarily of PEC filmsand metal contacts, can be made of the materials described previouslyfor 708 and 709. In some embodiments, the photoelectrocatalytic filmsare thin metal oxide films and the metal contacts are metals and/orelectrically conducting polymers. Although for simplicity only fourphotoelectrocatalytic electrodes are shown in FIG. 17, it is to beunderstood that any number of catalytic films and contacts can be usedto facilitate selectivity in measuring various analytes and/or otherpurposes. For additional selectivity, one or more analyte-selectivemembranes composed of polymers, dielectrics, ion-selective electrodematerials and/or particle size filters can be deposited on one or moreof the PEC electrodes. These materials are commercially available andwell known to those skilled in the art.

The package 1701 itself may be compatible with TO-header, Bergquistand/or similar standardized packaging arrangements. In some cases, thesubmount layer 1709 and secondary mounting layer 1710 are used, as theoptical source 1703 is part of the monolithic photoelectrocatalyticarray 1700. In other embodiments, layers 1709 and 1710 can beinterchangeable or, in some cases, the submount 1709 need not be used.For example, the bond pad metallization of a flip-chip LED 1703 can belined up with metallization patterned directly on a Bergquist thermalpackage—including a circuit layer, dielectric material, and metallicbase material—such that a submount layer may not be used. In many cases,the submount 1709 and secondary mounting 1710 can be integrated into oneunit to reduce the thermal resistance between the two layers. Thecontrol electronics 1711 may include powering, amplifying, signalconditioning, signal transmitting, signal processing, analog-to-digitalconversion, digital-to-analog conversion and/or similar electronics.These control electronics may be located near the bottom of the sensor1700 (as shown in FIG. 17) but other locations may be used. In somecases, the control electronics are not all included in the sensor 1700but rather are at least partially bifurcated from the sensor.

Various other embodiments of FIG. 17 also may be provided. For example,in FIG. 17, the PEC electrodes 1708 are shown facing away from theoptical excitation 1705. In these embodiments, the PEC films in the PECelectrodes may be sufficiently thick to absorb sufficient opticalenergy, but not so thick as to absorb all of the photons close to thePEC surface facing the optical emitter 1703. If the PEC layer is toothick, it may be difficult for free charge to reach the PEC surfaceclosest to the analyte 1707. Embodiments that can reduce or avoid thiseffect can invert the PEC electrode array 1708 shown in FIG. 17 suchthat the PEC electrodes face the semiconductor light emitting source. Apathway may be provided for the analyte between the PEC electrode array1708 and the semiconductor light emitting source 1703. This pathway canbe achieved by simply cutting a hole in the support layer 1712 and/orcreating a gap. In these embodiments, an additional protective opticallytransparent layer, such as a quartz window, may need to be mountedbetween the semiconductor light emitting source 1703 and the PECelectrode array 1708 to hermetically protect the semiconductor lightemitting source.

A partial top-view of a photoelectrocatalytic sensor 1700 is shown as1800 in FIG. 18. Only the photoelectrocatalytic films 1808, the contactlayers 1806, and the support layer 1802 are shown for clarity. Note thatexemplary rectangular geometries are drawn for the photocatalyticelectrodes, but other shapes, such as circles, squares, triangles and/orother shapes are also suitable. Moreover, although the contact layers1806 are shown on top of the photoelectrocatalytic films 1808, thesecontacts can be placed at the edges, the center, the corners and/orvirtually anywhere along the photoelectrocatalytic film surface. It maybe desirable to provide enough space between the contacts to allowdetection of absorbed analytes near the surface of thephotoelectrocatalytic films 1808. Also, although only two contacts perphotocatalytic film are shown, more contacts could also be employed inother embodiments. For example, in some cases, a 4-point-probearrangement may be used for monitoring conductivity changes,photocurrent and/or photovoltage, in the PEC film.

A variety of PEC element configurations can be implemented according tovarious embodiments of the invention. In some embodiments 1900 (FIG.19), interdigitated metal contacts 1906 are patterned onto the PEC films1908. The interdigitated metal contacts may allow conductivity,capacitance, photocurrent and/or impedance to be monitored in the samedevice structure more readily than with simpler electrodes. In somecases, the metal electrodes of each interdigitated pair may bedissimilar metals. For example, one metal may form a rectifying contactwith the PEC film and the other may form an ohmic contact. For theinterdigitated PEC electrodes 1900 of FIG. 19, a reference region ofelectrodes can be generated by depositing an insulating opticallytransparent film (such as SiO2, SiN, TiN, TaN etc.) over some electrodeswhile leaving other electrodes exposed to analyte. In this way, thephotoconductive response of the reference electrodes can be deconvolutedfrom the photoconductive+photoelectrocatalytic response of the exposedelectrodes. This reference electrode may be part of the contact layer709.

A “flip-chip” monolithic PEC sensor 2000 integrated into a flip-chip LED(or LD) die according to other embodiments of the invention isillustrated in FIG. 20. In this sensor 2000, the PEC elements and theLED source are monolithically integrated onto two different sides of thesame substrate, which can simplify packaging and/or back-end alignment.In these embodiments, PEC electrodes may be fabricated on thetransparent substrate-side (“backside”) of a flip-chip LED die. Forexample, GaN UV LEDs may be grown epitaxially on the polished“frontsides” of transparent sapphire substrates, and the sapphirebacksides are available for the deposition of metal oxide catalyst filmsand the fabrication of PEC electrodes through standard photolithographictechniques well known to those skilled in the art of microlithography.Optical energy 2005 from the LED 2003 passes through the transparentsubstrate 2007 and excites the PEC electrodes 2008 at the surface. Asanalyte at the PEC electrode surface (not shown in FIG. 20) is ionizedby the PEC effect, a net electrical charge forms at the electrodesurface. This charge can be monitored through electrical contacts 2006on each PEC electrode 2008 that may be connected with wirebonds to a setof connector pins 2015. The wires of the wirebonds are not shown in FIG.20 for simplicity and clarity. The entire monolithic PEC sensor 2000 canbe packaged onto a flip-chip board 2010 such as a Bergquist board, FR-4,alumina and/or the like. An interface layer 2009 may be provided toconnect the metal contacts of the LED 2003 to the flip-chip board 2010.In some cases, the interface layer 2009 may also include a siliconsubmount or the like, although this may increase the thermal resistancebetween the LED and the package.

FIG. 21 illustrates an “epi-up” monolithic PEC sensor 2100 according tostill other embodiments, where the PEC film 2108 also serves aspassivation for the LED epitaxy 2103. Because light 2105 leaves the topof the LED as opposed to the bottom through the substrate 2107, theremay be no need to provide mounting layers, such as 2010 of FIG. 20, forelectrical contact. The mounting layers for sensor 2100 can be virtuallyany solid layer that is sufficiently thermally conductive. A TO-header,printed circuit board, ceramic board and/or Bergquist board could eachserve as a mounting structure. The n-(2118) and p-(2117) contacts of theLED epitaxy 2103 may be isolated by a mesa pattern in the LED epitaxy.Mesa patterns are typically formed by standard microlithographicdry-etching techniques, such as reactive-ion etching or plasma-etching.Methods for fabricating GaN-based LEDs are well known. The n- and p-bondpads 2116 may be metal layers serving to connect the contact layers withexternal electronics for electrical communication and/or powering. Thebond pads 2116 may be deposited after etching windows through thepassivation layer 2108 via wet- and/or dry-etching techniques. In someembodiments, the PEC film 2108 can serve as both the PEC catalyst aswell as the dielectric passivation layer, all in one film. In otherembodiments, the PEC film 2108 may cover a separate passivation layeralready covering the LED epitaxy 2103.

As with the flip-chip PEC sensor of FIG. 20, the epi-up PEC sensor ofFIG. 21 may be thought of as a traditional LED structure with theaddition of a monolithically integrated PEC film. This PEC film can bedeposited by well-developed deposition methods such as plasma-enhancedchemical vapor deposition (PECVD), low pressure chemical vapordeposition (LPCVD), magnetron sputtering and/or the like. Additionally,more experimental deposition techniques, such as modulated-laserdeposition, can be used to ablate metal oxide targets for epitaxialgrowth of metal oxide films on a substrate layer, such as sapphire.

PEC sensors according to embodiments of FIGS. 20 and 21 can be used tosense volatile analyte near the PEC surface. Analyte near the surface isionized (reduced or oxidized) following photo-excitation of the PEC film(2008 and/or 2108). This imparts a net charge on the surface which canmodulate the p-n junction depletion region inside the LED epitaxy (2003and/or 2103). An example of the band diagram structure for a monolithicPEC sensor in the presence of a reducible analyte is presented in FIG.28.

The band diagram of FIGS. 28A and 28B show the p- and n-region depletionregion only, as it is assumed that the PEC film covering the LED surfaceis highly insulating and not generating a significant depletion regionin the p-doped epitaxy. Before analyte reaches the monolithic PEC sensorsurface, the LED is running under normal operational bias (shown in FIG.28A). Following the exposure to a reducible analyte species,photo-excited electrons are injected into the analyte, reducing theanalyte, and leaving a net positive charge on the PEC film, as shown inFIG. 28B. This net charge increases the junction voltage drop,increasing the forward LED impedance. As a non-limiting example ofmeasuring the PEC response shown in FIGS. 28A and 28B, a biasing circuithaving a series impedance, Z, and a regulated constant voltage source,V, can be used, as shown in FIG. 28C. The voltage drop across Z, shownas V_(M), will be a function of the voltage drop seen at the LED, whichis in turn a function of the analyte PEC response. These embodiments canprovide for monitoring various analytes with a monolithic PEC sensor anda simple circuit. A similar circuit can involve an oscillating voltagesource, and the insertion loss of the circuit can be measured as afunction of the capacitive reactance of the Z/LED series. A variety ofother monitoring circuits can be used which are well known to thoseskilled in the art. For example, an oxidizable analyte may leave a netnegative charge on the PEC film, thereby lowering the LED impedance.

A capacitive PEC element according to other embodiments of the inventionis illustrated in FIG. 22. The PEC film 2208 accumulates charge in thepresence of analyte and photo-excitation 2205. This charge is imaged onthe conductive plates 2206 of the capacitor. These electrode plates cancomprise any conductive material, such as metal, conductive polymer,doped semiconductors, conductive ceramics and/or the like. A dielectricmedium 2220 exists between the conductive plates to form a stablecapacitance between the plates. The charge imaged on the plates isdirectly proportional to the amount of ionized analyte, and this chargecan be measured electrically and/or optically. In some embodiments, thetop plate of the conductive plate electrodes 2206 contains no metal, butrather, the PEC film 2208 itself can serve as the top plate.

A cross-section (side view) of a simple interdigitated PEC electrode2300 according to some embodiments of the invention is shown in FIG. 23.A top-view was shown in FIG. 19. In these embodiments, the conductiveelectrodes 2306 are located on top of the exposed PEC film 2308. Opticalexcitation 2305 from the underlying optical emitter 2303 generatesphotocatalysis in analyte near the PEC film surface. As with the otherPEC structures, the optical emitter 2303 may be a monolithic LED or LD,or may be a complete packaged optical source, such as the packaged LEDshown in FIG. 12 and/or FIG. 27.

FIGS. 24A and 24B illustrate piezoelectric PEC elements 2400 accordingto other embodiments of the invention. In these embodiments, opticalphoto-excitation 2405 generates a charge at the exposed PEC surface 2408only in the presence of analyte. Charge from the PEC film 2408 generatesa field between the electrically conductive electrodes 2406 on eitherside of a piezoelectric crystal 2422. This field mechanically actuatesthe piezoelectric crystal via the piezoelectric effect, and thismechanical deflection can be detected electrically, magnetically,optically and/or acoustically.

For example, in the presence of ionizable vapors, if the UV LED ismodulated near the resonant frequency of the piezoelectric crystal, thepiezoelectric crystal will mechanically resonate as net chargeaccumulates and dissipates along the PEC film. This mechanical resonancecan be detected remotely by an acoustical sensor tuned to the resonantfrequency of the piezoelectric crystal. In piezoelectric monitoring,either or both the amplitude and frequency of the electromechanicalvibrations can respond to the PEC effect in the presence of analyte, andthis vibrational response can be related to the analyte type andconcentration. The mechanical resonance can also be detectedelectrically through the piezoelectric electrodes 2406. As a furtherexample, optical energy directed toward the piezoelectric crystal willbe scattered or modulated in relationship to the mechanical motion ofthe piezoelectric crystal. FIGS. 24A and 24B show piezoelectric PECsensors of different configurations. In FIG. 24A, the conductiveelectrodes are mostly contiguous, whereas in FIG. 24B, the conductiveelectrodes have at least one opening for photo-excitation 2405 to passthrough the piezoelectric crystal 2422 and through the PEC film.

A surface-acoustic-wave (SAW) PEC sensor electrode 2500 according to yetother embodiments of the invention is illustrated in FIGS. 25A and 25B.For simplicity and clarity, the PEC photo-excitation is not shown inthis figure, though it should be understood that photo-excitation, forexample from a semiconductor optical emitter integrated into the PECsensor, is used for triggering the PEC effect. In the embodiments shownin FIG. 25A, the driving electrode 2506 generates a surface acousticwave traveling towards the receiving electrode 2507 in the approximatedirection shown by the arrow 2505. A PEC film 2508 on the same substrate2522 as the driving and receiving electrodes, is located between thesetwo electrodes. The substrate 2522 should be sufficiently piezoelectricto generate and propagate a SAW wave at the surface. For example,quartz, aluminum nitride, piezoelectric lead zirconium titanate (PZT)and/or other piezoelectric solid state films may be used as SAWsubstrates. In response to volatile analyte near the photo-excited PECfilm 2508 surface, a net charge accumulates in the PEC film, and thischarge can affect the intensity, speed and/or propagation of the SAWwave along the piezoelectric substrate 2522. These affected SAWproperties are received by the receiving electrode 2507 and converted toelectrical energy, via the piezoelectric effect, providing a method ofsensing the presence and/or concentration of volatile analyte near thePEC film 2508 surface.

In FIG. 25B, the PEC film 2526 is one of the driving electrodes (2526and 2525) such that the SAW signal is generated by the PEC effectitself. In these embodiments, charge generated in the presence ofanalyte species and photo-excitation generates a voltage between the PECfilm electrode 2526 and the metallic electrode 2525. To facilitate thisprocess, the metallic electrode 2525 can be grounded or set at nominalfixed potential. The voltage generated by the PEC effect in turngenerates a SAW signal that propagates towards the receiving electrode2507. The receiving electrode then converts SAW signal into anelectrical signal as previously described. In both FIGS. 25A and 25B, itshould be noted that these PEC SAW electrodes can be arrayed withmultiple PEC films as illustrated in the figures, thereby providing aPEC sensor 2500 that can have enhanced specificity to particularanalytes.

Other embodiments of the invention that can sense various volatileanalytes by combining thermocatalysis with photocatalysis areillustrated in FIGS. 26A and 26B. Two separate embodiments are outlinedin FIGS. 26A and 26B. In FIG. 26A, the PEC electrodes 2608 are arrayedon the surface of a substrate 2609. A thermal gradient 2610 is appliedin at least one direction along the PEC electrode array 2608. Asemiconductor optical source generates photo-excitation 2605 to triggerthe PEC effect in the presence of volatile analyte (not shown). Becausevolatile vapors are generally characterized by different catalytic andsurface interaction properties at different temperatures, some vaporswill interact more readily with PEC electrodes at hotter or coldertemperature. Thus, the PEC electrode signal intensity may be greater insome electrodes as opposed to others, and this difference can be used toenhance the specificity of the PEC electrode array 2600 to particularanalytes, such as VOCs. In these embodiments, the individual PECelectrodes of the PEC electrode array can be the same catalyst and stillprovide analyte specificity. Again, this is because of thetemperature-dependent interaction of VOCs with a given catalyst, suchthat interaction for some VOCs is stronger at higher temperatureswhereas for other VOCs this catalytic interaction is weaker withincreasing temperatures. In embodiments of FIG. 26A, the PEC effect canbe used to sense these temperature-dependent interactions. The potentialbenefits of using the PEC effect over the thermocatalytic effect forsensing these interactions has been described above and will not berepeated for the sake of brevity.

The thermal gradient 2610 of FIGS. 26A and 26B can be generated withlocalized filament heaters, monolithic patterned resistors, a resistiveheater, a graded filament heater, a radiative source and/or the like. Insome cases, the gradient may be provided by positioning a filamentheater on one end of the substrate layer 2609, such that the temperaturedecreases with distance away from the heater. Providing a thermalgradient may consume significant power to generate the heat, but not asmuch as that consumed for the pure thermocatalysis technique.

FIG. 26B illustrates other embodiments that combine thermocatalysis andphotocatalysis where a thermal gradient is not needed. In theseembodiments, thermocatalysis is provided by optical radiation, such asIR radiation 2611, directed towards the PEC electrode. The higher theoptical radiation 2611, the higher the surface temperature of the PECelectrode. Thus, the temperature of each PEC electrode can beindependently regulated by optical excitation from a separate opticalheating source. In some embodiments, a single UV PEC excitation sourceis integrated with multiple IR sources (such as IR LEDs), one IR sourcefor each PEC electrode. In other embodiments, the PEC films in the PECelectrodes are doped with an optically absorbing impurity that absorbslight at one or more sub-band-gap wavelength(s), turning the opticalenergy into thermal energy. Different doping methods in neighboring PECfilms can result in different thermal heating rates and thus provide adifferent surface temperature for each PEC film. In yet otherembodiments, the PEC electrodes 2608 are located on films that generatethermal energy in response to a particular wavelength of light. In yetother embodiments, a single UV source, a single thermal radiationsource, and a single PEC electrode may be employed for specifyingbetween multiple volatile analyte species. This may be achieved bycycling the thermal radiation source in the presence of UV excitationsuch that the PEC effect is monitored in the PEC electrode duringthermal cycling. Because each VOC generally has a unique thermalcharacteristic, this thermal characteristic can be monitored by the PECeffect as a function of both temperature and time and used to identifythe presence and intensity of various VOCs and other volatile species.One or more arrays of multiple PEC excitation sources, multiple thermalradiation sources and/or multiple photocatalysts can be employed tofurther specify one analyte species from another. Additionally, one ormore “white” or multiwavelength sources can be used in conjunction witha prism or other refracting or diffracting source to generate multiplewavelengths from the same optical source.

The electrodes of FIGS. 22-26 may serve as the PEC electrodes 1708 ofthe PEC sensor 1700 of FIG. 17. The electrodes of FIGS. 22-26 may alsoserve the more general PEC elements (comprised of 708, 709 and 707) forthe PEC sensor embodiments 700 of FIG. 7. In general, it should beunderstood that any of the PEC electrode configurations described hereincan be used in any of the PEC sensing devices and methods describedherein. Additionally, the electrodes can take other configurationsaccording to other embodiments of the invention. For example, the PECfilm can be part of the gate in a gated electronic device, such as thegate of a field-effect transistor (FET), a gated diode, a gatedphotodiode and/or the like. Thus, the PEC film can be part of anelectronic device having 3 or more terminals. Furthermore, rather thanforming entire new PEC sensors, the PEC films can be formed oncommercially available device wafers, device die, or fully packageddevices and systems. For example, one or more PEC films can beselectively deposited or selectively patterned over a commerciallyavailable metal-oxide semiconductor field-effect transistor (MOSFET), aheterostructure field-effect transistor (HFET), resistor, diode,photodiode and/or the like, such that the PEC film covers at least oneactive terminal of the device. For example, the PEC film can bedeposited directly on the gate contact metallization of a commerciallyavailable FET or the contact metallization of a commercially availableresistor or diode. In yet other embodiments, a PEC element can beintegrated at least partially with a microelectromechanical system(MEMS) device.

The broad range of PEC electrode configurations is exemplified by thegated FET structure 2900 of FIG. 29. The source 2921 and drain 2918contacts of the FET epitaxy 2903 are isolated by a mesa patternunderneath the gate layers 2917. The source 2921 and drain 2918 contactsare typically composed of one or more metals, and in some cases thesecontacts cover specially doped layers of epitaxy 2903 to promote currentflow. Mesa patterns are typically formed by standard microlithographicdry-etching techniques, such as reactive-ion etching or plasma-etching.The methods for fabricating FET structures are well known to thosehaving skill in the art. The source and drain bond pads, 2916 and 2919respectively, may be metal layers serving to connect the source anddrain contacts, 2921 and 2918 respectively, with external electronicsfor electrical communication and/or powering. The gate bond pad 2920 maybe connected with external electronics, connected with a referenceelectrode, or unconnected and left electrically floating. In someembodiments, the gate bond 2920 pad may not be used. There may also be apassivation layer (not shown in FIG. 29) covering part of the device,and this passivation layer is typically an oxide, nitride, orcombination of oxides and nitrides, but any insulating dielectric can beused. Typically, these dielectrics should not generate charge with UVlight; for example, it generally is desired that the band gaps for thesedielectrics should be greater than the energy of the UV excitationphotons. The substrate layer 2907 is typically a solid crystalline orpolycrystalline material, as described elsewhere herein. In the FETstructure 2900, the gate layers 2917 are primarily used to modulate theconductance of one or more channels in the FET epitaxy 2903 between thesource 2921 and the drain 2918 contacts. The thickness of the films inFIG. 29 typically ranges from a few tens of Ångstroms to severalmicrons.

The gate layers 2917 are shown as 3 separate layers in FIG. 29 tohighlight an embodiments of a PEC gated structure 2900. In the gatelayers 2917, at least one layer is composed of at least one PECmaterial. In other embodiments, at least one layer is composed of atleast one PEC material and at least one layer is composed of at leastone dielectric material. In yet other embodiments, at least one layer iscomposed of at least one PEC material and at least one layer is composedof at least one metallic material. In still other embodiments, the gatelayers 2917 consist of simply one layer—the PEC layer. In otherembodiments, only two layers are used. In other embodiments, more thanone type of PEC material exists in the gate layers 2917. Moreover,although the FET structure 2100 is illustrated as a MOS-type structureon a single-crystal substrate, it should be understood that a MOS-typestructure on a flexible substrate, a polymer substrate, organic materialand/or other material can be used in other embodiments of the invention.Additionally, other transistor configurations, such as bipolar junctiontransistor configurations, can also contain PEC material for modulatingthe net charge or conductivity of the emitter, collector, base, or otherlayers.

Thus, gate layer(s) 2917 may be configured in a variety ofconfigurations to allow the gate to function as a PEC-enabled gate. Forexample, PEC charge, in response to analyte and photo-excitation at/nearthe PEC surface, can be imaged onto a metallic film, a dielectric film,or the FET epitaxy directly. This PEC charge can modulate theconductivity of the conductive channel between the source and drain inproportion to the concentration of analyte at the gate 2917 surface.Different gate layer 2917 configurations can provide control over devicecapacitance, speed, selectivity and/or reliability. Additionally, in thegate layers 2917 of FIG. 29, at least one PEC material may be integratedor dispersed within a dielectric or metallic film, providing additionalflexibility in PEC sensing. For example, the gate layers 2917 may be asingle layer composed of a simple metallic film with regions of PECmaterial dispersed throughout the film. These PEC materials can besputtered or deposited into a metallic film at various depositiontemperatures. In some cases, at least one component of the gate layers2917 is composed of at least one nanostructure. This component may alsobe composed of at least one PEC material.

In other embodiments of the invention, an additional electrode may beprovided to modulate, regulate, regenerate and/or balance charge nearthe surface of the PEC film. For example, oxidized or reduced analytenear the PEC surface may reside near the surface, and this may generallybe undesirable. Because the lingering analyte may possess charge that isequal to the charge in the PEC surface, but opposite in polarity, thenet charge at the surface may be at or near zero. An additionalelectrode near the PEC surface can reduce or eliminate this effect. FIG.30 shows a 3-electrode PEC capacitor 3000 comprising a charge-balancingelectrode 3030 on the PEC capacitor 2200 of FIG. 22. The balancingelectrode 3030 is shown as three layers to emphasize that this electrodemay be comprised of one or more layers. In some embodiments, thereference electrode comprises an electrically insulating layer adjacentto the PEC film 3008 and an electrically conducting layer adjacent tothe insulating layer. In other embodiments, a third layer adjacent tothe electrically conducting layer provides electrochemical protection tothe balancing electrode 3030. For example, the insulating layer may bean insulating dielectric layer, the conducting layer may be a metalliclayer, and the electrochemical protection layer may be a native oxide tothe conducting layer and/or another dielectric layer. In otherembodiments, the balancing electrode 3030 may be comprised of aconducting layer only, or an insulating and conducting layer only. Othercombinations may be used as well. Photoexcitation 3045 may be providedby a semiconductor light emitting source according to variousembodiments of the invention described herein.

FIG. 31 shows how a circuit comprising the 3-electrode PEC capacitor3000 may operate for sensing a voltage response to a photocatalyzedanalyte near the PEC surface. Multiple balancing electrodes, eachcomposed of at least one insulator and one conductor, are arrayed acrossthe surface of the PEC film. The insulator of the balancing electrodemay be used to electrically isolate the conductor of the balancingelectrode from the underlying PEC film and the underlying capacitorstructure. Oxidized positively charged analyte (shown as positivelycharged circles) from photocatalysis are attracted to the balancingelectrode via electrostatic potential. This leaves a net negative chargenear the PEC film that alters the potential across the capacitorelectrodes, and this can be measured electrically through a voltmeteracross the capacitor electrodes. In these embodiments, the balancingelectrode can provide a net negative charge at the surface that can bemeasured as a voltage through a voltage detecting circuit (V). Without abiased balancing electrode, positively charged oxidized analyte mayreside at the surface, attracted to the net negative charge in the PECfilm, providing a zero or near-zero net charge near the PEC-analytesurface. Thus, a change in potential may be difficult to observe or maynot be observed in the presence of photocatalyzed analyte.

The balancing electrode 3030 can be fabricated via standardmicroelectronic photolithography. For example, a dielectric film (theinsulating film) and conductive film can be deposited, in sequence, overthe PEC film. The conductive film and dielectric film can then bepatterned via wet- and/or dry-etching techniques. A variety ofgeometries can be used for the balancing electrode 3030, such asrectangles, squares, circles, triangles, interdigitated fingers, etc.,in various embodiments of the invention. Though the balancing electrode3030 is shown, for exemplary purposes, along with a PEC capacitordevice, it should be noted that the balancing electrode 3030 can be usedin virtually any device configuration, such as a diode, transistor,light-emitting device and/or other devices as described herein.

In other embodiments, a biasing technique can be used to clear thesurface of charged analyte even without a balancing electrode 3030. Forexample, a positive potential bias between the top and bottom electrodes2206 of the PEC capacitor 2200 can repel positively charged analyte nearthe PEC surface. Following the electrostatic deflection of analyte fromthe surface, the net negative charge in the PEC film can be measured. Avariety of other electrostatic and/or electrodynamic charge balancingtechniques can be used to facilitate a net charge in the PEC film forvoltage measurements in response to photocatalyzed analyte. Othertechniques for cleaning the surface of charged analyte can includethermal pulsing and/or electromagnetic excitation of the surface. Forexample, an infrared source, such as an IR LED, can be modulated anddirected upon the PEC surface to energize the bound analyte andencourage the removal of charged analyte bound at the surface.

In other embodiments of the invention, a PEC diode 3200 can be used todetect analyte fluids. Some diode embodiments are illustrated in FIGS.32A and 32B. Referring to FIGS. 32A and 32B, a PEC film 3208 is adjacentto at least one electrical contact 3206 of a PEC diode 3200.Photoexcitation 3205 stimulates photocatalysis at the PEC surface,leaving a net charge in the PEC film which can modulate the space chargeregion in the diode structure 3203. Modulation of the space charge canbe detected as a voltage or current response across the electricalcontacts 3206 of the diode. The vertical PEC diode of FIG. 32A mayoperate with optical excitation on the top-side whereas the lateral PECdiode of FIG. 32B may operate with optical excitation from theback-side. For PEC excitation from the backside of diode of FIG. 32B,the diode structure 3203 may need to be sufficiently transparent to thephotoexcitation light 3205. Additionally, the electrical contact 3206adjacent to the PEC film 3208 may need to be sufficiently transparent tothe photoexcitation light 3205.

The PEC diode structures 3200 of FIGS. 32A and 32B are examples and donot limit embodiments of this invention to simple diodes. For example,the PEC detection methodology of FIG. 32 can be applied towardsvirtually any device where at least one space charge region can bemodulated. This may be the case for the gate of a field-effecttransistor, the gate of a silicon-controlled rectifier, the terminalsacross a p-n junction and/or other device structures. Additionally, theability to detect very low analyte levels can be achieved for the casewhere the diode structure 3203 is that of an avalanche photodiode (APD).In such case, a net charge generated in the PEC film, in response tophotocatalyzed analyte, can be amplified by the internal avalancheamplification process of an APD. Additionally, a PEC film can beprovided on the photocathode of a photomultiplier tube or semiconductorphotomultiplier device for analyte detection via photocatalysis. Thedesign and fabrication of diodes is well known to those skilled in theart, and the deposition and patterning of PEC films on electronicdevices has already been described herein.

Methods for fabricating PEC sensors, such as shown in FIG. 12, accordingto various embodiments of the invention, will now be described. Thesemethods will highlight fabrication processes for a monolithic flip-chipLED PEC sensor (FIG. 21). However, many other processes may be used.

In particular, in some embodiments, AlInGaN LED epitaxy is grown on theepi-ready “frontside” of a crystalline, optically transparent c-planesapphire substrate via commercialized metal-organic chemical vapordeposition (MOCVD). The term “frontside” refers to the side of thesubstrate on which the LED epitaxy is primarily grown, and the term“backside” refers to the side of the substrate which is largely free ofLED epitaxy. The total LED film thickness may range from about 2 toabout 4 μm to reduce strain in the epitaxy, but thicker films are oftenused. For AlInGaN, n-type layers may be grown first, followed by theactive region (for generating light) and p-type layers. Thus,considering the example shown in FIG. 21, 2117 is usually a p-contactand 2118 is usually an n-contact.

Following MOCVD epitaxial growth, the LED wafer may be introduced into aplasma deposition or modulated laser deposition (PLD) tool, fordepositing the metal oxide photocatalysts on the epi-free backside ofthe sapphire wafer. Magnetron sputtering of the photocatalysts, asopposed to PLD, may also be used. Other deposition techniques are alsosuitable. During PLD, the frontside of the LED wafer can be protected byfacing the frontside away from the PLD growth zone or by covering thefrontside with a protective “sacrificial oxide,” such as SiO₂, to besubsequently removed. PLD targets can be generated by sintering powderedmixtures of oxides, such as gallium oxide, magnesium oxide, zinc oxideand/or aluminum oxide. Sometimes, these oxides can be mixed withrare-earth oxides, such as Eu₂O₃ for more specialized optoelectroniceffects (FIG. 16C). During PLD, these targets may be ablated by anexcimer laser under high vacuum (˜5×10⁻⁸ torr), and the by-productssettle on the substrate to form a contiguous, crystalline oxide film.Film growth can be performed at a wide range of temperatures, but about700-900° C. may be used in some embodiments. A temperature should bemaintained that will not destroy the AlInGaN epitaxy, which is primarilyon the frontside of the substrate. Multiple catalyst films can bedeposited sequentially by changing the PLD target inside the PLDdeposition tool. This can be automated during PLD growth without havingto open the PLD reactor or break vacuum. For example, a gallium oxidetarget can be ablated first, followed by a magnesium oxide target, thenfollowed by a praseodymium-doped gallium oxide target, etc., yielding alayered luminescent film of the same order structure as the ablatedtarget procession.

In some cases, as in the case of FIG. 12, the photocatalytic films maybe deposited on a transparent substrate, such as quartz or sapphire,that is not part of the AlInGaN LED. This process can reduce concernsassociated with multiple oxide deposition runs on a wafer covered withsensitive AlInGaN epitaxy.

It should be noted that growth conditions during PLD, magnetronsputtering, MOCVD, and the like may play a large role on the opticalquality of the deposited films. For example, growth at highertemperatures may encourage high-crystalline growth and may supportdopant incorporation at optically active lattice sites. Furthermore, bynanoscale engineering, nanostructures can form quantum dots and/orquantum wells which can further support radiative recombinationefficiency and overall brighter optical output. For example, depositingthin, nanoscale (e.g., <100 nm) films of lower band gap materialsurrounded by higher band gap material can result in higher radiativerecombination efficiency within the lower band gap film. The filmdeposition processes described herein, and methods of incorporatinghigh-brightness nanostructures, shall be regarded as non-limitingexamples, and may be modified by those skilled in the art withoutdiverging from the intent of the invention. Furthermore, the morphologyof the metal oxide surface can dramatically affect surface adhesion,electrostatic attraction, analyte diffusion, adsorption/desorptionand/or other types of physical interactions between the analyte and themetal oxide photocatalyst.

Following growth, the resulting wafer can be processed as a standardAlInGaN LED, using photolithography steps well known to those skilled inthe art. However, if photocatalysts are deposited onto the backside thebackside of the substrate of the LED wafer, as in 2000 of FIG. 20, thebackside may need to be protected throughout processing. Processing maystart with a solvent clean of the wafer surface, sometimes followed byan acid clean. The photocatalyst films on the sapphire backside can beprotected by utilizing a sacrificial layer of protective photoresistduring acid cleans and oxide etching steps. This sacrificial layer istypically removed before annealing steps. Following the clean, selectivep-contacts may be formed on the surface of the AlInGaN epitaxy, followedin order by mesa formation, n-contact metallization formation, contactannealing, surface passivation, and bond pad formation. Typicalp-contact metallizations are nickel oxide-gold alloy (NiO/Au), nickel,platinum and/or silver. Typical n-contact metallizations are Ti/Al, Aland/or Ti alloys. Metallization can be formed via standarde-beam/thermal evaporation or sputtering techniques. Mesa formation istypically executed by dry-etching approaches, with chlorine-basedchemistries, such as reactive ion etching (RIE) and/or inductivelycoupled plasma (ICP) etching. The mesas may be formed to aid with lightextraction and to define regions for the n-contact layer. The etchedregions may serve as the n-contact interface whereas the mesa tops mayserve as the p-contact interface, for the n- and p-contactmetallization, respectively. The p-contact layers may be metallizedfirst, prior to mesa formation, to protect the delicate p-type epitaxyfrom the detrimental effects of RIE or ICP etching steps. Surfacepassivation may be executed by the sputtering or plasma-enhancedchemical vapor deposition (PECVD) of silicon dioxide and/or siliconnitride, although other dielectrics are also possible. In the monolithicPEC sensor of FIG. 21, the surface passivation is itself thephotocatalyst film. In such case, metal oxide films may be depositedprimarily by plasma deposition, laser deposition and/or magnetronsputtering. Ti/Ni/Au bond pads may then be patterned by selectivelyetching holes in the passivation layer along the n- and p-metal contactregions. These bond pads may serve as the location for wirebonds duringsubsequent LED packaging.

If the substrate backside contains an array of photocatalytic films,these films can be fabricated into PEC electrodes through the standardfabrication steps described above, but in this case the frontside of theLED wafer may need to be protected. For PEC electrodes, a one-stepmetallization mask can be used to pattern metallic electrode featuresinto the PEC electrodes. In the case of FIG. 12 where the PEC electrodesare fabricated on separate substrates from the AlInGaN LED, there may bemore flexibility in the fabrication process. For example, multipledeposition runs can be used to deposit multiple catalysts, without theneed to protect the front-side of the substrate, and these catalysts canbe selectively etched via dry-etching techniques such that separate,noncontiguous photocatalysts remain. Metal electrodes can then bedeposited and patterned on each photocatalyst using the samephotolithographic mask. These metal electrodes may be patterned in avariety of geometries, such as interdigitated electrodes, as shown inFIG. 12.

The fabricated LED wafer can be diced or sawed to generate hundreds tothousands of LED die per substrate, typically 1 mm² in area. These diemay then be flip-chip bonded to a silicon submount, and the submount maybe mounted to thermal packaging (for heat extraction). In someembodiments, the submount is not used and the LED die may be attacheddirectly to the thermal packaging, with the AlInGaN epitaxy facing down(towards the thermal packaging) and the luminescent film facing up (forlight extraction). With the LED fabrication process, sawing, dicing, andpackaging can be executed with standard recipes well known to thoseskilled in the art. Care may need to be taken to protect thephotocatalysts that may rest on the backside of the wafer. This can bedone by applying a protective tape over the photocatalyst to preventscratching and/or other mechanical damage during sawing, dicing,packaging, and other back-end processes. Of course, for PEC electrodeson an independent substrate, such as that shown in FIG. 12, theseconcerns may not exist, as the LED and the PEC electrodes can beprocessed separately.

Following die separation, the packaged LED die can be mounted on aTO-header. If PEC electrodes are not already on the LED backside, aseparate transparent substrate with patterned PEC electrodes can bemounted with the TO-header package (as shown in FIG. 12). Standard goldwirebonds can connect the LED and PEC electrodes with the TO-header. Theentire packaged PEC sensor can then be integrated with a telemetricprocessing board, such as a Bluetooth module, for wireless communicationwith a cell phone, computer, or PDA. The device processes describedherein are non-limiting examples, and can be modified extensively bythose skilled in the art without diverging from the intent of theinvention.

Other embodiments of the invention include methods of monitoring fluids706 using, for example, a photoelectrocatalytic sensor and real-timemonitor 700 of FIG. 7. These fluids 706 may be gases and vapors fromexhaled breath, airborne and waterborne contaminants, industrialpollution, airborne volatile compounds and/or the like. Monitoringfluids from each case may involve different sensor designs. For example,a respiration monitor may be mounted on a portable headpiece or amouthpiece. In a specific case, a standardized portable phoneheadset—including a headpiece, an earpiece, and mouthpiece—canincorporate one or more photoelectrocatalytic sensors in the mouthpiece.Such a sensor can facilitate oximetry, capnometry and/or healthdiagnoses.

A photoelectrocatalytic sensor used as an environmental monitor may belocated virtually anywhere. For example, an industrial pollution monitormay be located at or near one or more exhaust ports. In some cases,pollution may come from a motor vehicle, in which case thephotoelectrocatalytic sensor 700 may be at or near the exhaust of thevehicle. In a specific case, a portable environmental monitor, composedof the photoelectrocatalytic sensor 700, can be incorporated in aportable wireless instrument such as an earpiece, headset, cell phone,PDA and/or the like. Typical environmental pollutants that may bemonitored include ozone, carbon monoxide, VOCs, PAHs and/or the like.Because photoelectrocatalytic sensors 700 do not need a heater filamentto initiate the catalytic reaction, these sensors can work well forwearable, human-portable sensors as well as explosion-proof sensors.

It should be noted that the photoelectrocatalytic layers 708 do not needto be layers that respond only to UV light to initiate photocatalysis.For example, indium oxynitride can have a band gap ranging from themid-IR to deep-UV. Thus, employing a photoelectrocatalytic layer 708 ofindium oxynitride in a photoelectrocatalytic sensor 700 can facilitatephotocatalysis via IR or visible light as well as UV light, depending onthe stoichiometry of the indium oxynitride alloy. Thus, an integratedphotoelectrocatalytic sensor 700 of indium oxynitride can incorporate anLED 703 in the visible or IR wavelengths as well as the UV. Moreover,nuclear radiation or X-rays can also be used to ionize the surface of aphotoelectrocatalytic sensor.

In the drawings and specification, there have been disclosed embodimentsof the invention and, although specific terms are employed, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims.

What is claimed is:
 1. A fluid analyte sensor, comprising: a firstphotoelectrocatalytic element that is configured to be exposed to afluid, if present, and to respond to photoelectrocatalysis of an analytein the fluid that occurs in response to impingement of optical radiationupon the first photoelectrocatalytic element; and a secondphotoelectrocatalytic element that includes an optically transparentlayer thereon that allows optical radiation to pass therethrough suchthat the second photoelectrocatalytic element is configured not to beexposed to the fluid, if present, but to respond to impingement ofoptical radiation upon the second photoelectrocatalytic element.
 2. Asensor according to claim 1 further comprising: a semiconductor lightemitting source that is configured to impinge the optical radiation uponthe first and second photoelectrocatalytic elements.
 3. A sensoraccording to claim 1 wherein the first and second photoelectrocatalyticelements each comprise a photoelectrocatalytic layer and at least oneconductive contact electrically connected thereto.
 4. A sensor accordingto claim 1 wherein the first and second photoelectrocatalytic elementseach comprise a plurality of spaced apart photoelectrocatalytic layersand a plurality of spaced apart conductive contacts electricallyconnected thereto.
 5. A sensor according to claim 1 wherein the firstphotoelectrocatalytic element is devoid of an optically transparentlayer thereon.
 6. A sensor according to claim 2 further comprising asubstrate, and wherein the first and second photoelectrocatalyticelements and the semiconductor light emitting source are at leastpartially monolithically integrated with the substrate.
 7. A sensoraccording to claim 1 wherein the fluid comprises a liquid and/or a gasand wherein the analyte comprises a pollutant, contaminant and/or acomponent of the fluid.
 8. A sensor according to claim 1 wherein thefluid comprises respired gas and wherein the analyte comprises acomponent of the respired gas.
 9. A sensor according to claim 2 whereinthe semiconductor light emitting source comprises at least one laserdiode and/or light emitting diode.
 10. A sensor according to claim 1wherein the first photoelectrocatalytic element is configured to respondto photoelectrocatalysis of at least one analyte in the fluid bychanging a conductivity, capacitance, inductance, impedance, net charge,optical property and/or mechanical property thereof in response to theimpingement of the optical radiation upon the firstphotoelectrocatalytic element.
 11. A sensor according to claim 1 whereinthe first photoelectrocatalytic element further comprises a transistor,a capacitor, a nanostructure, a diode, an amorphous structure, apiezoelectric structure, a surface acoustic wave structure and/or aheating element.
 12. A sensor according to claim 2 wherein the firstphotoelectrocatalytic element and the semiconductor light emittingsource include at least one common electrical contact.
 13. A sensoraccording to claim 2 wherein the semiconductor light emitting sourceincludes a passivation layer and wherein the first and secondphotoelectrocatalytic elements are at least partially on the passivationlayer.
 14. A sensor according to claim 2 further comprising aphotodetector that is configured to detect the optical radiation that isemitted by the semiconductor light emitting source and/or opticalradiation emitted by the photoelectrocatalysis.
 15. A sensor accordingto claim 2 further comprising a controller that is configured toselectively energize the semiconductor light emitting source, to detecta response to the photoelectrocatalysis of at least one analyte in thefluid that occurs in response to impingement of optical radiation uponthe first photoelectrocatalytic element, and to deconvolve a response tothe impingement of the optical radiation on the secondphotoelectrocatalytic element from the response to thephotoelectrocatalysis of the at least one analyte in the fluid thatoccurs in response to the impingement of the optical radiation upon thefirst photoelectrocatalytic element.
 16. A sensor according to claim 2wherein the first photoelectrocatalytic element is doped with deep levelimpurities and wherein the semiconductor light emitting source comprisesat least one visible and/or infrared light emitting diode and/or laserdiode.
 17. A sensor according to claim 2 wherein the firstphotoelectrocatalytic element comprises a metal oxide, wherein thesemiconductor light emitting source comprises an ultraviolet and/or bluelight emitting diode and/or laser diode.
 18. A sensor according to claim1 further comprising a monitor that is configured to monitor theresponse to the at least one analyte in the fluid, if present, resultingfrom the photoelectrocatalysis of the at least one analyte in responseto the impingement of the optical radiation upon the firstphotoelectrocatalytic element, and to monitor the response of the secondphotoelectrocatalytic element to the impingement of the opticalradiation thereon.
 19. A sensor according to claim 1 further comprisinga monitor that is configured to monitor energy of the at least oneanalyte in the fluid, if present, resulting from thephotoelectrocatalysis of the at least one analyte in response to theimpingement of the optical radiation upon the firstphotoelectrocatalytic element, and to monitor the response of the secondphotoelectrocatalytic element to the impingement of the opticalradiation thereon.
 20. A sensor according to claim 3 wherein thephotoelectrocatalytic layer comprises a plurality of layers of a givenphotoelectrocatalytic material having at least two different impuritiestherein.
 21. A sensor according to claim 3 wherein thephotoelectrocatalytic layer comprises a plurality of layers of differentphotoelectrocatalytic materials.
 22. A sensor according to claim 3wherein the photoelectrocatalytic layer comprises oxide, carbide,nitride, arsenide, phosphide, sulfide and/or antimonidephotoelectrocatalytic compounds and/or metal oxides, metal nitrides,metallic compounds and/or semimetallic compounds thereof.
 23. A sensoraccording to claim 3 wherein the at least one conductive contactcomprises at least two interdigitated conductive contacts.
 24. A sensoraccording to claim 15 wherein the controller is configured to repeatedlymodulate the semiconductor light emitting source and to detect anelectrical response of the first and second photoelectrocatalyticelements in response thereto.
 25. A sensor according to claim 15 furthercomprising a wireless transmitter that is responsive to the controller.26. A sensor according to claim 18 wherein the monitor is configured tomonitor an electrical, electromagnetic, mechanical, acoustic and/orthermal response to the at least one analyte in the fluid.
 27. A sensoraccording to claim 19 wherein the monitor is configured to monitor theenergy as optical energy of the at least one analyte in the fluid.